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Training in the Heat: How Not to Cook Yourself This Summer

Greg · UpGreg AI Coach Published August 3, 2026 4 min read

Every summer, same standoff: your training plan on one side, the thermometer on the other. And every summer, runners insist on holding April paces in the middle of July, wonder why their sessions feel “bad,” and end up head-first in the canal — literally and figuratively. Heat isn't something you fight. It's something you manage.

What heat actually does to your body

Above roughly 25 °C, your body is running two jobs at once: producing movement AND dumping heat. Some of your blood is diverted to the skin to cool you down, which leaves less for your muscles: at the same pace, your heart rate climbs by 10 to 20 beats per minute. That's not a drop in fitness, that's physics.

The consequence is counter-intuitive: forcing the prescribed pace means training harder than the plan asked for. Your easy run turns into a threshold session, your threshold session turns into a race. The fatigue shows up, the progress doesn't — and that's how a summer of “hard work” leaves you in poor shape come September.

The rules of summer

• Run by feel or by heart rate, not by pace. At the same effort, your summer pace is 20 to 40 s/km slower. Accept it: the training adaptation is exactly the same.
• Pick your hours: before 9am or after 8pm. A 1pm session in August isn't commitment, it's stubbornness.
• Drink before you're thirsty — and past the one-hour mark, carry fluid (500 ml/h as a rule of thumb, more when it's brutal).
• Move your intensity: intervals in the middle of a heatwave are the fastest way to turn a quality session into a heat casualty.
• Adapt the route: forest, riverside, a short loop close to home. Shade beats elevation gain in full sun, every time.

Acclimation: two weeks, and your body genuinely changes

The good news is that your body learns. Within ten to fourteen days of regular exposure, you start sweating earlier, more heavily and with less salt lost, and your plasma volume increases. Translation: the same run costs you far less at the end of July than it did during the first hot spell.

Worth remembering before you draw conclusions: the first hot runs are always the worst. That's not the moment to judge your fitness, or to rewrite your plan. It's also a good reason not to discover heat on race day: if your goal race falls in summer, deliberately put a few sessions in the hot hours during the two weeks before it.

The signs that mean stop

Shivering in full sun, goosebumps, sweating that suddenly stops, headache, confusion, nausea: these are no longer discomfort signals, they're heat stroke signals. Stop, get into the shade, cool down (water, ice, wet cloth), drink, and don't hesitate to call for help. No session is worth it.

And if your numbers are off all week, check your training load before blaming the weather: CTL, ATL and TSB often say what fatigue won't. On the recovery side, water and a bit of salt after a heavy sweat are part of the meal, not an optional extra: see recovery starts at the table.

These guidelines are for a healthy adult recreational athlete. Exertional heat stroke is a medical emergency. If you take medication, have a heart condition or are pregnant, talk to a health professional before training in high heat.

🗣️ Greg's take

“In summer I recalibrate your effort budgets automatically: your heart rate tells me what the thermometer is putting you through, and your plan adjusts. If your session reads 6:10/km instead of 5:30 at the same heart rate, you haven't gone backwards — you just ran in a furnace. And for cooling down, I recommend the shower. Not the water pistol on café terraces.”

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