Three weeks into last summer’s heatwave I ran what my watch called an easy 8K at what my heart rate called a threshold session. Pace was fine — the same low-6s-per-km I’d been running all spring. Heart rate was 15 beats above where it usually sits for that effort, and it did not come down. I finished the run overheated, irritable, and slower on the second half than the first, for no reason except that I had asked my body to do a cool-weather run in 29°C with nothing changed. That was the day I stopped trusting pace on hot days and started trusting effort.
Heat is load, not weather
The mechanism is simple enough that it is worth just stating plainly: running generates heat, and in warm conditions your body has to divert blood to the skin to shed it — blood that would otherwise be carrying oxygen to your legs. Core temperature climbs, perceived effort climbs with it, and the only lever left is to slow down. Endurance performance peaks around 10–12°C; above that the penalty starts small and gets steep. Skoma’s marathon heat calculator puts rough numbers on it — about 1% slower near 15°C, 3% near 20°C, 6% near 25°C, and 10% or more by 30°C — and those figures match what a hot easy run feels like from the inside: fine for the first kilometre, then a slow-motion negotiation.
The point isn’t the exact percentage. It’s that heat behaves like a training stress in its own right, stacked on top of whatever session you’d planned. Ignore that and you’re not running your plan in the heat — you’re running a harder, uncounted version of it.
What I change: pace by effort, not by the watch
The single biggest adjustment is dropping pace as the number I steer by and picking up heart rate instead. On a hot day I run to a heart-rate zone and let pace fall wherever it falls — sometimes 20–30 seconds per kilometre slower for what should be the same easy effort. I know roughly where my zones sit from Skoma’s heart rate zones calculator, so on a hot morning I’m not guessing at a number, I’m just refusing to let the pace on my watch override what my heart rate is already telling me. Heart rate drifts up before pace falls apart — by the time pace visibly slows, you’re already behind on the adjustment.
What I change: when the run happens
Second adjustment, and the easiest one: runs move to dawn. Not because early miles are magically cooler in some meaningful physiological sense, but because in most places the gap between 6 a.m. and 2 p.m. is the difference between “warm” and “actively dangerous,” and that gap is free. I have moved long runs as early as I could stand to get up for, and moved hard sessions to whatever window in the day is coolest — sometimes that’s dawn, sometimes it’s after sunset. What I don’t do is run a threshold session at midday in a heatwave and call it dedication. That’s not toughness, it’s just a worse session for no benefit.
What I change: hydration starts the day before
By the time you’re thirsty on a hot run, you’re already behind, and no amount of mid-run drinking fully closes that gap. So hydration for a hot session starts the evening before, not with the bottle I carry. That’s a habit shift more than a science project — drink properly through the day before a hot long run, and don’t treat the run itself as the only opportunity to hydrate.
What I change: acclimatisation gets a real two weeks
The first week of a heatwave is always the worst, and that’s not bad luck — it’s the body not yet having adapted. Sweat rate, plasma volume and the point at which you start sweating all shift over roughly one to two weeks of consistent heat exposure, and until that happens every run feels disproportionately hard for the effort you’re putting in. I’ve learned to expect that window and not panic inside it: the session that feels broken in week one of a heatwave often feels completely normal by week three, with nothing else about my fitness having changed. The mistake is reading week one’s bad session as a fitness problem instead of an acclimatisation one.
What I refuse to change: the sessions that need to be moved indoors, or skipped
Here’s the part I used to get wrong: treating every session as sacred and adjustable, when some sessions are neither. A VO₂-max interval set at 35°C in full sun isn’t a harder version of the workout — it’s a different, riskier activity wearing the same name on the training plan. My rule now is blunt: if a session’s purpose depends on hitting a specific intensity and the heat makes that intensity unsafe, the session either moves onto a treadmill or gets skipped outright, not slogged through outdoors at a pace that technically matches the number but costs three times the physiological toll. A skipped interval session is a non-event. A heat-related medical one is not, and I’ve had exactly one scare — dizzy, nauseous, stopped a session early on a day I should have moved indoors — that was enough to make this rule permanent.
Heat is a training load in itself. Pretending your 10K pace still applies is how summer injuries happen.
What I don’t change is the actual training plan underneath all of this — the weekly structure, the long-run progression, the easy-day mileage. Heat changes how a session is executed, not what the block is trying to build. I still use Skoma’s pace calculator to know what a given effort should look like in normal conditions, so I have a real baseline to compare against once the temperature drops back down and I can tell the difference between “this got harder because of the heat” and “this got harder because I’m actually undertrained.” That comparison is the whole reason to keep effort-based data through a heatwave instead of just writing the weeks off: it tells you, once the heat breaks, exactly how much of the season you protected.