In a nutshell 💡
Preparing for a fall marathon means training in the middle of summer. The method comes down to five key steps: adjust your pace by 10 to 20 seconds per kilometer, run early in the morning or in the evening, stay hydrated with electrolytes, acclimate yourself over 7 to 14 days, and wear appropriate temperature-regulating gear. The heat then becomes a manageable factor, not an obstacle to your goal.
You’re running for October, not for July. But it’s in July that you win your marathon.
That’s the paradox for runners aiming for a fall race: the most crucial weeks of training (when you build long-distance endurance) fall right in the middle of summer. July, August. Long runs. Interval training. Back-to-back sessions. All of this in temperatures of 30, 33, sometimes 36 °C. This guide gives you practical tools to ensure that summer isn’t a gap in your training, but a full-fledged step toward your marathon.
Understanding what heat really does to your training
Why Your Performance Automatically Slows Down Above 20 °C
The optimal temperature for endurance performance is between 6 and 8 °C. Above that, every additional degree affects your performance. At 25 °C, the loss in performance is estimated at 3%. At 30 °C, it rises to 8–10%. At 35°C, it exceeds 12%.
The mechanism is simple: to dissipate the heat generated by exercise, your body diverts some of the blood flow to the skin to trigger sweating. The result: less blood reaches the muscles, so less oxygen is available for exercise. Your speed drops. This isn’t a weakness; it’s physics.
Heart rate drift: the signal you ignore at your own risk
At a steady pace in intense heat, your heart rate gradually increases over the course of the session. This is heart rate drift. For an effort that seemed like basic endurance at the start, your heart rate can climb by 10 to 20 beats per minute after an hour of running.
What started as a recovery run turns into a threshold effort. Your body temperature rises, and your perception of effort skyrockets. Trying to maintain your planned pace under these conditions means exhausting yourself unnecessarily and jeopardizing your next session.
Dry or humid heat: two different challenges
Running in 32°C dry air is nothing like running in 28°C by the sea. In dry heat, sweat evaporates quickly and cools the skin effectively. In humid heat, the saturated air blocks this evaporation: sweat flows but no longer cools you down.
The key figure to watch isn’t just the thermometer reading, but theheat index, which combines temperature and humidity. This is what should guide your decision to go out, adjust, or postpone a session.
Warning signs to watch for before calling it quits
Your body sends clear signals before reaching its limit. Learn to recognize them during your run:
- Stopped sweating despite exertion: a strong sign of advanced dehydration.
- Paradoxical chills in intense heat: thermoregulation is beginning to fail.
- Headaches, nausea, dizziness: the nervous system is overwhelmed.
- Repeated cramps: massive loss of salt and minerals.
- Confusion or difficulty maintaining your pace: an absolute emergency—stop immediately.
⚠️ Important note: two simultaneous signs mean you must stop immediately, no exceptions. Move into the shade, splash cool water on the back of your neck and wrists, and drink in small sips. Heatstroke is a medical emergency. It cannot be managed by continuing your workout.
See also: Heat Waves and Exercise: How to Keep Training Safely
Adjust your pace when the temperature exceeds 28°C
Pace Adjustment Chart by Temperature
Here is a practical guide for adjusting your speed in the heat. These guidelines apply to moderate humidity and a runner who is gradually acclimated.
| Perceived temperature | Pace adjustment per km | Impact on long sessions |
|---|---|---|
| Up to 20 °C | No adjustment | Normal session |
| 20–25 °C | +5 to +10 sec/km | Monitor hydration carefully |
| 25–30 °C | +10 to +15 sec/km | Reduce duration or split session |
| 30–35 °C | +15 to +20 sec/km | Prefer short runs morning/evening |
| Above 35 °C | Postpone or train indoors | High risk: do not push too hard |
Train by heart rate zones, not by target pace
In the summer, basing your training on pace is counterproductive. The heat causes your heart rate to rise even at a constant pace: you’re overworking yourself without any additional benefit. The best approach is to switch to heart rate zones.
Simple rule: if your heart rate exceeds your usual zone ceiling by more than 10 beats, slow down. Your goal is to maintain the duration of the effort, not the speed. Your aerobic metabolism will still improve.
The best times to train and those to avoid
The temperature distribution of a summer day follows a predictable pattern. To maximize the effectiveness of your workouts:
- 6 a.m.–8 a.m.: the ideal window in summer, with the lowest temperatures and relative humidity still at an acceptable level.
- 7 p.m.–9 p.m.: a viable alternative; the ground is still radiating heat, but the air is cooling down.
- 11 a.m.–5 p.m.: absolutely avoid this time for any long or intense workouts during a heatwave.
For these morning slots exposed to the rising sun, your cooling t-shirts make all the difference: UPF 50+ to block UV rays, active sweat wicking to maintain your thermal comfort zone.
💡 Practical tip: never plan a long summer run the night before if the next day’s forecast exceeds 30°C. The disconnect between your training plan and the actual heat conditions is the number one reason people drop out of marathon training. Be the manager of your plan, not its blind follower.
Learn more: Guide to UPF 50+ certified clothing: how to choose based on your activity
Acclimating to the heat: the key factor most runners overlook
What acclimatization does to your body
Thermal acclimatization is not a metaphor. It is a real, measurable physiological adaptation that occurs after 7 to 14 days of gradual exposure to heat stress.
Its concrete effects on your performance:
- Increase in blood plasma volume (+3% to +8%): greater capacity to transport oxygen.
- Earlier onset of sweating: cooling begins before core body temperature rises too high.
- Reduced salt concentration in sweat: fewer mineral losses.
- Decrease in heart rate during the same level of exertion by 3 to 7 beats per minute.
The 3-phase acclimatization protocol
To achieve these adaptations without putting yourself at risk, follow this progression:
Phase 1, days 1 to 5: short exposures, 30 to 45 minutes of moderate exercise in a hot environment. Goal: to acclimate the cardiovascular system without depleting reserves.
Phase 2, days 6 to 10: Gradually increase duration up to 60–75 minutes. You can reintroduce light intervals, always while monitoring your heart rate.
Phase 3, days 11 to 14: Normal sessions with pace adjustments. Adaptations are now active. You can resume structured sessions.
During this gradual build-up, targeting vascularized areas accelerates adaptations: the nape of the neck, wrists, and forehead. cooling accessories play a role in thermal support here, not just comfort.
⚙️ Technology explained: the Jadecool technology range is based on jade microcrystals integrated into the fabric fibers. When in contact with moisture (sweat or manual activation), they accelerate evaporation and produce a cooling sensation up to 15°C below ambient temperature. This cooling effect remains active for several hours depending on ambient temperature, without the need for battery recharging or consumables. It’s physics applied to sports apparel.
Hydration and Nutrition: How Heat Changes Your Calculations
Calculating Your Actual Needs in Summer Heat
In cool weather, a runner’s fluid loss is around 0.5 to 1 liter per hour of exertion. Above 28°C, this figure rises to 1.5 or even 2 liters per hour, depending on intensity and body type.
Two simple guidelines to adjust your strategy:
- Weighing before/after your run: every pound lost = approximately 1 liter of fluid deficit to make up.
- Urine color: pale yellow = good hydration. Dark yellow or amber = a deficit to correct before your next run.
Water alone or with electrolytes: the 60-minute rule
For efforts lasting less than 60 minutes, water alone is sufficient in most cases. Beyond that, and especially in hot conditions, water alone is no longer enough. You lose sodium, potassium, and magnesium through sweat. Drinking only water dilutes these minerals in the blood: this is hyponatremia, an underestimated complication among marathon runners.
For runs lasting an hour or more in intense heat, opt for a slightly salty drink or supplement with an electrolyte tablet. Simple, low-calorie, and effective.
Nutrition during your run: what changes in the summer
Heat suppresses appetite and slows digestion. Standard energy gels can be hard to tolerate above 30°C. A few practical adjustments:
- Reduce the amount of carbohydrates per serving, increase the frequency of servings.
- Choose liquid formats (isotonic drinks) over solid gels.
- Test your nutrition strategy during training before applying it on race day.
“Whether in competition or training, an athlete who doesn’t hydrate properly in intense heat jeopardizes their performance long before they feel thirsty. Thirst is already a delayed signal. »
— Gwenaël Fournet, Product Expert at G-Heat
At the end of a session in hot weather, cooling towels help limit the buildup of heat fatigue: placed on the back of the neck or forearms for a few seconds, they speed up the return to balance.
Cooling gear: what you wear makes all the difference
Why Your Choice of Fabric Affects Your Workout
The fabric you wear matters. Cotton traps sweat and feels heavy. Standard polyester doesn’t effectively wick away body heat. Technical fabrics designed for sports in hot weather work on two fronts: rapid sweat wicking and protection from the sun’s rays.
For summer marathon training, the two top priorities are:
- UPF 50+ protection: blocks over 98% of UV rays, essential for long runs in direct sunlight.
- Active cooling: accelerated evaporation technology keeps your skin cooler than standard fabric.
The T-shirt: the foundation of your summer kit
The men’s cooling sports T-shirt and the women’s cooling sports T-shirt feature TechCool™ technology. The breathable technical weave accelerates sweat wicking without manual activation and keeps the fabric active throughout your session, from morning interval training to long runs.
For longer runs in the low morning sun, the men’s long-sleeve cooling anti-UV T-shirt in JadeCool™ extends coverage to the forearms and blocks UV rays thanks to its UPF 50+ certification.
The head and neck: the areas you overlook
The head is a major heat exchange surface. Exposing your head to the sun accelerates the rise in body temperature. Two accessories make all the difference on long runs:
The cooling sports headband featuring TechCool™ technology keeps your forehead dry without being too tight. It absorbs sweat and regulates the temperature of your forehead, an area with a particularly dense network of blood vessels.
The cooling sports cap featuring PolyCool™ technology combines shade with a long-lasting cooling effect thanks to moisture-retaining fibers. Ideal for outings lasting over 90 minutes in direct sunlight.
PolyCool™ technology is available across the entire range of cooling caps: from the running cap to the full-brimmed safari hat for total sun protection.
The neckband: small accessory, big thermal impact
The back of the neck is a highly vascularized area. Cooling the back of the neck is equivalent to cooling the blood that supplies the brain. The JadeCool™ cooling neckband becomes damp in 30 seconds and keeps this contact point cool for several hours. It is often the most effective accessory for reducing the perceived effort during the final 20 kilometers of a long summer training run.
“The rule is simple: if you go out for a run lasting more than an hour in temperatures above 28°C, you must have addressed the heat issue before setting out. It’s not an option—it’s a prerequisite for performance.”
— Romane Benderradji, Communications Manager at G-Heat
Must-read: How to Sleep Well During a Heat Wave: Recovery and Gear
Structuring Your Training Week When the Heat Sets In
The Weekly Schedule Adapted for Summer
The temptation is to stick to your training plan as is, hoping your body will keep up. It’s understandable. It’s risky. A typical week tailored for marathon training in extreme heat looks more like this:
- Monday: active rest or mobility (no running).
- Tuesday: short intense session (45 min max), early in the morning, below 24°C.
- Wednesday: active recovery, slow jog for 30–40 min, in the evening.
- Thursday: quality session, interval or threshold training, in the morning, duration reduced by 15%.
- Friday: rest or mobility.
- Saturday: long run, at sunrise, duration adjusted according to the expected heat index.
- Sunday: light active recovery or rest.
When to postpone a session without feeling guilty
Postponing a session doesn’t mean giving up on your plan. It means managing it intelligently. Conditions that justify immediate postponement:
- Perceived temperature above 35°C for the entire available time slot.
- Humidity index above 32°C (combined heat and humidity).
- Less than 6 hours of sleep the previous night due to extreme heat (impaired thermal recovery).
- A long session planned and deteriorating weather over the next 48 hours.
Use summer conditions as specific training
A tough session completed at 30°C produces cardiovascular adaptations you cannot replicate under ideal conditions. The heart rate drift you manage in the summer teaches you to train based on how you feel, not on numbers. This is exactly the skill you’ll need at the 35-kilometer mark of a fall marathon if the weather is against you.
For Saturday’s long run, cooling your neck before setting out is often the most effective move for the final 20 kilometers. The G-Heat range of cooling neck wraps covers all sizes, from the compact model to the long-lasting JadeCool neck wrap.
Summer isn’t your enemy. It’s your unwitting thermal trainer.
Why choose G-Heat to prepare for your marathon in extreme heat
G-Heat has been designing thermoregulation gear since 2017 for everyone who can’t stop when it’s hot.
- Field-tested technologies: JadeCool™, TechCool™, and PolyCool™—three complementary active ingredients that cover all summer training scenarios, from morning interval training to long runs in direct sunlight.
- Performance gear, not just comfort: each product is designed to integrate seamlessly into athletic movement without restriction: lightweight, activates in 30 seconds, machine washable.
- Honest approach: G-Heat is a thermoregulation equipment manufacturer, not a therapist. The gear supports your comfort zone. It does not replace hydration, first aid, or stopping a workout when your body demands it.
- Continuity of activity: the goal is not to ignore the heat. It is to allow you to continue preparing for your fall goal even when July and August work against you.
Frequently Asked Questions About Summer Marathon Training
Can you really make progress by running in intense heat?
Yes, provided you adjust your pace. The cardiovascular adaptations produced by exercising in the heat (increased plasma volume, better sweat regulation) are real and transferable to the cooler conditions of your fall marathon.
How many seconds per kilometer should I add for each additional degree?
Above 20°C, add about 5 seconds per kilometer for every additional 5°C. In practice, at 30°C, slowing down by 15 to 20 seconds per kilometer compared to your baseline marathon pace is reasonable and protective.
Does heat acclimatization fade quickly before the race?
The adaptations gained during acclimatization begin to fade after 2 to 3 weeks without exposure. If your marathon is in late September or October, a final phase of exposure in August is enough to maintain the benefits.
Should you run in a running jacket even in the summer?
No, on the contrary. In the summer, the goal is to maximize the skin’s surface area for heat exchange. Fewer layers, but the right ones: a technical, temperature-regulating UPF 50+ t-shirt, a cap, and a neck gaiter. Lightweight and breathable fabrics are key.
My heart rate is always higher in the summer—is that normal?
Yes. A higher heart rate at a steady pace in the summer is physiologically normal. Blood is diverted to the skin to ensure cooling, which increases the heart’s workload. This isn’t a sign of poor conditioning; it’s a normal adaptive response.
Is hydration with electrolytes really necessary for runs under 2 hours?
For runs under 60 minutes, water is usually sufficient. Between 60 and 120 minutes in very hot weather, incorporating a source of sodium becomes helpful to prevent electrolyte dilution and maintain muscle contraction. For runs over 2 hours in the heat, electrolytes are no longer an option.
Cap or headband: which one should you choose for running in the summer?
The two serve different purposes. A cap protects against direct sunlight on the head and face. A headband manages sweat from the forehead and maintains visual comfort. For a long run in full sun, combining both is the most comprehensive solution. For a short run in cooler weather, the headband alone is sufficient.
Whatever the weather. G-Heat, in all weather conditions.
Sources
[1] “Heat and athletic performance: how can we limit the risks involved?”, Weekly Epidemiological Bulletin No. 7, Santé publique France
[2] “Heat and athletic performance”, Santé publique France
[3] “2024 Paris Olympics: Heat Wave Hits Sports”, INSERM
[4] “Guide to a Sporty and Responsible Summer”, Ministry of Sports
[5] “Heat Stroke: Symptoms and What to Do”, Ameli.fr
[6] “Tips for Running in Extreme Heat”, French Athletics Federation
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