Quick Answer: Heat slows running pace by roughly 2 to 4 seconds per mile for every degree above 60°F, and humidity compounds it. Train by effort or heart rate through August, not by pace, move long runs to the earliest possible start, and expect your race-day pace to be 20 to 40 seconds per mile faster than your August training pace once temperatures drop. The heat is doing real training work — plasma volume and sweat efficiency both improve — so the summer build isn’t wasted, it’s just unreadable by pace.

Week 9 of a 14-week plan says 8 miles at goal pace. It’s 82°F with 70% humidity at 7am, and you finish 45 seconds per mile slow, feeling like the goal was fantasy.

It wasn’t. You just tried to read a hot-weather run with a cool-weather instrument.

How much does heat actually slow you down?

The commonly used figure is a 2 to 4 second per mile penalty per degree above about 60°F, and it accelerates rather than staying linear.

At 70°F, expect roughly 2% slower. At 80°F, 4 to 6%. At 85°F with high humidity, 6 to 10% is entirely normal for a well-trained runner. For a 9:00/mile runner, that’s a 9:30 to 9:55 pace at the same effort.

Humidity is the bigger villain and gets less credit. Sweat cools you only when it evaporates. Above about 70% humidity, evaporation slows dramatically, so you sweat heavily and get very little cooling from it. Dew point above 65°F is the threshold where most runners feel the wheels come off.

Heart rate drifts upward at constant pace — cardiac drift — because blood volume shifts toward the skin for cooling and less returns to the heart per beat. Seeing your heart rate climb 10 to 15 bpm across a long run at unchanged pace is expected in heat, not a fitness signal.

None of this means your fitness is worse. It means the environment is taxing a system that has nothing to do with your aerobic ceiling.

How should I adjust the plan?

Convert every pace-based workout into an effort-based one, and keep the duration rather than the distance.

Easy runs: run by breathing. You should be able to speak a full sentence. In heat, that will be considerably slower than your usual easy pace, and that’s correct — the physiological stimulus is the same.

Long runs: keep the time, let the distance fall. A 100-minute long run at whatever pace the day allows is a better stimulus than forcing 12 miles and finishing depleted. Time on feet is the training variable that matters most.

Tempo and threshold work: use heart rate or a pace conversion. Add the heat penalty to your target — a 8:00/mile threshold becomes 8:25 at 80°F. Or run by feel: comfortably hard, sustainable for an hour if you had to.

Intervals: move them indoors or move them earlier. Short, hard efforts suffer most in heat because there’s no time to shed the load between reps. A treadmill session is not a compromise; it’s the correct tool for a 90°F week.

Build in one extra recovery day in the hottest fortnight. Heat training carries a recovery cost that a 14-week plan written in a spreadsheet doesn’t account for.

What does heat acclimation actually give me?

Real, measurable adaptations in 10 to 14 days — and most of them help on a cool race day too.

Plasma volume expands 4 to 15% within the first week or two of consistent heat exposure. More blood volume means better cardiac output and better cooling capacity, and that benefit doesn’t disappear when the weather cools.

You start sweating earlier and more efficiently, and your sweat becomes less salty as the body conserves sodium. Both mean better temperature regulation at lower physiological cost.

Resting and exercising heart rate at a given pace drop as acclimation develops. If your August heart rate at easy pace is lower in week four than week one at similar conditions, acclimation is happening.

The adaptation is fast to gain and fast to lose — roughly two weeks to build meaningfully, and it fades within one to two weeks without exposure.

This is why a summer build into a fall race often produces personal bests. You did the hard training under a handicap, and then the handicap was removed on race day. The typical improvement when temperatures drop 20°F is 20 to 40 seconds per mile at the same effort.

5 Hot-Weather Training Mistakes That Cost Fall Races

1. Chasing the paces on the plan anyway. Running goal pace in 85°F means running at threshold effort during what should be an easy or moderate session. Do that for six weeks and you arrive at the taper overreached, which is the most common way a fall race goes wrong.

2. Under-drinking on long runs because carrying water is annoying. Sweat rates in heat commonly reach 1 to 1.5 liters per hour. A 90-minute long run with no fluid is a substantial deficit, and the effects on the following days’ training are larger than the effect on that run.

3. Ignoring sodium entirely. Water alone in high sweat conditions dilutes blood sodium. For runs over 60 minutes in heat, aim for 300 to 700mg of sodium per hour alongside fluid. This is the variable most runners never touch and it fixes a surprising number of late-run headaches and cramps.

4. Scheduling the long run for Sunday morning regardless of the forecast. Move it. A Saturday 6am long run in 68°F is worth far more than a Sunday 8am one in 84°F, and there’s no rule that says long runs happen on weekends.

5. Racing a tune-up 5K in peak heat and drawing conclusions. A hot tune-up race gives you an unreliable number that then dictates your goal pace. Either adjust it with a heat calculator or don’t use it for prediction at all.

What changes in the last four weeks as it cools?

This is when you finally get to see what you built, and there’s a specific way to check it.

Wait for a cool morning and run one honest workout at goal pace. Somewhere in the 3 to 4 weeks out window, when conditions drop below 65°F, do 4 to 6 miles at goal race pace. That run is your real fitness reading — everything before it was through a filter.

Expect it to feel surprisingly manageable. Runners who have trained through a hot summer routinely find goal pace easier than they feared, because they’ve been running that effort all along at slower speeds.

Set your goal after that run, not before it. A goal chosen in July based on spring fitness and then defended through a hot August is a goal disconnected from evidence.

Keep some heat exposure in the taper if race day might be warm. October races can still deliver a 72°F morning. One warm run a week maintains acclimation cheaply.

Adjust fueling for the cooler day. You’ll drink less and need less sodium than your training taught you. Practice race-day fueling on the cool days, not the hot ones, or you’ll take in far more fluid than you need.

FAQ: Hot Weather Half Marathon Training Questions, Answered

Should I use a treadmill for all my summer runs?

No — some heat exposure produces the acclimation benefit that pays off later. A reasonable split is easy and long runs outside in the coolest part of the day, and hard interval sessions inside when it’s dangerous outside.

How early is too early to start a long run?

Whatever time gives you the coolest conditions and adequate sleep. A 5:30am start after seven hours of sleep beats a 7:30am start after six. If the early start costs you sleep, the trade isn’t worth it.

How do I know if it’s heat exhaustion rather than just a hard run?

Chills, goosebumps, stopping sweating, confusion, nausea, or a headache that appears mid-run are all warning signs. Stop, get to shade or air conditioning, drink cool fluids with sodium. If symptoms don’t resolve quickly, seek medical help.

Will my race pace really be that much faster?

A 20°F temperature drop typically produces 20 to 40 seconds per mile at the same effort, and heat-acclimated runners often see more. This is one of the reliable pleasant surprises in fall racing.

This is general information, not medical advice.

TL;DR: Your summer training isn’t slower because you’re unfit. It’s slower because it’s summer.

• 2 to 4 seconds per mile slower per degree above 60°F, and humidity compounds it well beyond that.
• Train by effort or heart rate, not pace. Keep long run duration and let distance fall where it falls.
• Heat acclimation is real training — plasma volume expands 4 to 15% in two weeks and carries into cool weather.
• 300 to 700mg sodium per hour on runs over an hour in heat, alongside 500 to 1000ml of fluid.
• Move the long run to whichever day is coolest. Weekend scheduling is a convention, not a rule.
• Set your goal after one cool-weather workout at 3 to 4 weeks out, not from a hot-day estimate.

Check the dew point rather than the temperature this week. Above 65°F, adjust the workout before you start it — that decision is worth more than any amount of toughness during it.