Summer is the season of overtraining.

Every year, when the temperature starts getting higher and the humidity begins to creep up, the coach in me begins to have little panic attacks. I’m not thinking about upcoming races, but about upcoming easy runs.

Why? Even after months of acclimation, running performance will still be negatively affected by heat and humidity. Many athletes struggle to recalibrate their easy effort for the heat of summer. Will my athletes accidentally go too hard when they are supposed to be recovering and building endurance?

Most of us don’t want to slow down as much as we must to adjust for hot weather—it feels like a sign of weakness, or even worse, fitness loss. But all else being equal, if your easy pace stays the same even as the conditions go from slightly balmy (i.e. California in summer) to oppressively swampy (i.e. Florida anytime), you will overtrain rapidly.

Anecdotal Evidence

One of my athletes, Amelia Boone (a top trail runner and obstacle-course racing champion), recently traveled from her home in California’s Bay Area to Long Island, a difficult trade anytime of year, but especially so during summer. On her last run in California, she ran eight miles with 1,200 feet of climbing at 7:40 pace and an average heart rate of 150 beats per minute, on a 66-degree morning. For her this was an easy day in the hills after a tough workout the day before.

On a similar run in Long Island (eight miles, 500 feet of climbing, 7:35 pace) in 88-degree heat, she averaged a heart rate of 162 beats per minute, with spikes up to 177 bpm. For Amelia—someone known to be a good hot-weather athlete—heat changed a recovery run into an unintentional workout.

My most striking experience with the heat/humidity buzzkill was during a run commute from my former office in Washintgon, D.C. in 2014. I was running at an aerobic heart rate of 153 bpm at 8:20 pace on a warm afternoon when a stiff wind blew through Rock Creek Park. Suddenly, as the trees swayed ominously, the temperature dropped nearly 30 degrees. Without any change in my effort or heart rate, I sped up by roughly two minutes per mile.

Related: The Basics of Heat Training for Running

Hot Weather Research

There is research to back up these anecdotes. A study in the journal PLOS One looked at 1.8 million marathon results and found that temperature was the most important variable predicting performance, with around a ten-percent drop in performance in hot weather versus optimal conditions. Other studies show similar performance decreases, magnified when it is humid.

A table complied by Runner’s Connect summarizes the science to lay out the stakes: dew point (a metric combining heat and humidity) as low as 60-65 degrees will impair performance two- to three-percent. Meanwhile, a typical summer dewpoint on the east coast—75-80 degrees—will impair performance 12-15 percent. Dew points above 80 degrees? They don’t even rank it.

This rapid decrease in pace relative to heat is caused by a host of factors, but the main culprit is increased blood flow through capillaries on the skin, for cooling, leaving less blood and oxygen for generating power.

However, the heat effect varies based on each runner’s body mass, physiology and acclimation status. Essentially, different people have different critical thresholds for the core temperature at which the body is forced to slow down substantially.

How to Keep “Easy” Easy

No matter how much you acclimate, or how fit you are, your performance will decrease in warm and/or humid summer environments. That is a hard pill to swallow for many runners. Isn’t summer supposed to be the time for fitness breakthroughs before big races?

As a result, many runners keep pushing, refusing to slow down as much as needed, often without realizing that they are lighting the fuse on a fatigue bomb. Every recovery run that’s a bit too hard shortens the fuse, until one day all that extra, unplanned stress will blow up in the form of illness, injury, burnout or one of countless other maladies.

How can you adjust your expectations to continue getting fitter when the weather gets hotter?

1. Check your ego at the door.

Guess what? Your physiology is the same as all of your fellow homo sapiens. All of us are affected by increased core temperature from hot-weather running.

Practice the mantra: “I am not slow because I am running more slowly. My easy pace is not a measure of my fitness.”

Prepare to have your easy pace increase by one-to-two minutes per mile in many instances. Take pride in your ability to show self-restraint. Most importantly, remember that “easy” is not a pace—it’s an effort. If your pace stays constant in heat, then you are actually going at a moderate or even hard effort. Moderate or hard running on easy days is how running progress stagnates and regresses over time.

2. Invest in a heart-rate monitor, or carefully calibrate your perceived exertion.

Heart rate is the most objective metric to account for external variables affecting running effort, including heat. For many of the athletes I coach, we use the Friel Method to calculate Lactate Threshold Heart Rate (LTHR) by taking the average heart rate in the last 20 minutes of a 30-minute time trial. For most athletes, around 85 percent of that number is a good cap for easy days (more experienced runners doing high mileage might be at 81-82 percent; newer runners a bit higher). You can also use a certain percentage of your max heart rate (usually 70-75 percent), or even the cruder Maffetone method of subtracting your age from 180 (which I don’t recommend unless you’re just starting out with heart-rate monitoring).

If you don’t believe in heart-rate monitoring, calculating effort is tricky but doable. Make sure that you aren’t looking forward to being done with your easy runs—it should feel like a pace you can hold for a really long time, maintaining conversations and singing songs. On a scale of one to 10, don’t go above four for purely easy recovery days and six for easy endurance days. And most importantly, slow down immensely on hills, since that extra stress can really make things difficult.

3. Make sure at least half of your running days are truly easy.

All year round—but especially during summer—apply the heart-rate or perceived-exertion rules for easy days at least half of the time. For example, if you’re running five or six days a week, three or more of those days must be easy.

On the other days, you can have at it. During summer, many athletes thrive on strides or slightly shorter intervals with longer recovery that let them go fast (maintaining or improving running economy) without getting burnt up by the heat. But, no matter what workouts you prefer, if you fail to give easy days the respect they deserve your body will come back to roundhouse kick you in the face.

Combine these strategies with heat-acclimation techniques, and you’ll stay healthy and happy during the swampy months ahead. With hot summer running, just showing up will lead to breakthroughs when it cools off. Don’t judge pace. Instead, focus on effort and patience.

David Roche runs for HOKA One One and NATHAN, and works with runners of all abilities through his coaching service, Some Work, All Play.