Why Slower Summer Runs Can Lead to Faster Fall Races

Most runners know heat slows them down. But believing those slow miles actually make them faster is a tougher concept to grasp.When your usual easy pace gets tougher, and the speed workout pace you hit in April now seems out of reach, it can lead to panic as your fall race gets closer.Andrew Evans, a Phoenix-based RRCA- and 80/20 Endurance-certified run coach, sees this concern firsthand. For example, one of his runners, who was training in Arizona before traveling to Minnesota for a half marathon, messaged him that she was worried all of her heat-affected running was making her slower.Evans knew the numbers on her watch told only part of the story. The effects of the heat were suppressing the pace she could outwardly achieve, even though her training was actually making her significantly stronger beneath the surface.The key to trusting that your fitness is still improving—even if your pace doesn’t show it—comes down to understanding why heat affects pace, how to adjust without sacrificing the workout, and the line between normal fatigue and more serious issues.Here, experts explain how summer’s frustratingly slow running actually builds a huge fall-race advantage.Why Your Normal Pace Feels Harder in the HeatAt a very basic level, all working muscles produce heat, explains Janet Hamilton, CSCS, exercise physiologist and run coach with Atlanta-based Running Strong. During the summertime, the environment adds external heat, forcing your body to work harder to maintain a healthy core temperature.“Your body recognizes [external heat] as a threat,” Hamilton says. In response, it sends more blood toward the surface of the skin—and away from the exercising muscles—as a cooling mechanism.Sweating cools the body, but mainly during the process of evaporation, Hamilton continues. If the surrounding air is hot and humid, evaporation is much harder to achieve. Sweat that doesn’t evaporate mostly just dehydrates runners without providing the desired cooling effect.During a run, this cooling process competes with the working muscles for blood, and the body prioritizes the cooling process, says Hamilton. Therefore, your heart works harder, your perceived effort rises, and your pace drops even though your underlying fitness hasn’t suddenly disappeared.Evans compares running in the heat with other forms of resistance training. You move slower running uphill, right? Heat creates a similar constraint on your pace.A general rule of thumb to remember is for every 5-degree increase above 60 degrees Fahrenheit, a runner can expect to add 20 to 30 seconds per mile to their typical pace. While the precise effect varies by runner and conditions, the point is, you can’t evaluate your summer pace as though the weather conditions were irrelevant.How Slow Summer Running Can Prepare You for a Fast Fall RaceThe single most important benefit of consistent heat training is increased blood plasma volume, Hamilton says, which basically means you have more blood circulating the body.Over time, as blood is used both to cool the body and work the exercising muscles during hot summer runs, the body adapts by increasing its plasma volume to distribute between the two demands, explains Hamilton.Then, when temperatures drop, the body no longer has to send as much blood toward the skin to stay cool. More remains available to deliver oxygen to the exercising muscles, allowing runners to exercise with greater efficiency. This means summer training quite literally can help you run faster in cooler races because it’ll feel easier to maintain that quicker clip.Like all physiological adaptations, this process requires consistency. Evans estimates runners need about two to three weeks of consistent outdoor training to fully acclimate to the heat and receive all the benefits. Completing as many of your workouts as possible outdoors presents your body with more opportunity to acclimate, whereas if you took to the treadmill for a few workouts a week, your acclimation period could be weeks longer.At the end of the summer season, Hamilton estimates a similar gradual decline period when temperatures begin to fall. She describes the ideal scenario as training all summer through hot weather and selecting an early fall race while you’re still operating with elevated plasma levels but the temperatures have leveled off from their summer peaks.Evans’ Arizona athlete took a slightly different approach, training in the sweltering heat but venturing up to cooler Minnesota for a half marathon in July. Despite feeling a lack of confidence in her summertime training going into the race, Evans’ runner crushed her stated time goal.Evans reports her successful race in a cooler environment restored her confidence in hot-weather training as she continues on to pursue a full marathon PR later this year.A slower pace in the heat does not necessarily mean a weaker training stimulus. At a comparable heart rate or rate of perceived exertion (RPE), a runner may cover less ground while still receiving the intended benefit of the workout. This is why Hamilton and Evans advise runners to prioritize running by effort when it’s hot outside.Consider a set of 400-meter repeats. In cool weather, you might run them at 5K race pace. On a hot day, Hamilton shifts the target to 5K race effort (8 or 9 out of 10 RPE). The number on your watch changes, but the purpose of the session does not.Runna’s new “Adapt for Heat” feature applies the same idea to its artificial intelligence–based training plans, adjusting targets using feels-like temperature, workout duration, and session type. In a press release from the company, Runna says 80 percent of its community reported adapting or skipping runs because of hot weather. This new feature automatically adjusts for that.“You can’t outwit Mother Nature,” Hamilton says. “Ease up the pace. Stay hydrated. If it’s a humid day, be willing to adjust even more.”What Happens When You Refuse to Slow DownForcing the same pace throughout the year changes the intensity of the session. An easy run can become moderate, while a controlled threshold workout can turn into an effort that demands far more recovery than your training plan intended. Repeated exposure to workouts well beyond their intended intensity can compromise every following workout, leading to a broken training plan.Evans says heart rate can help runners control the intensity of relaxed runs, although factors beyond heat such as caffeine, sleep, stress, and even time of day can affect the number. RPE is often the best metric for determining whether you are still running at the intended effort level. On an easy run, Evans recommends sticking to a 3 or 4 out of 10 RPE.Continuing to force the same pace also accelerates the consequences of exercising in the heat. Your heart rate climbs higher, dehydration can set in quicker, and you’ll find yourself well on your way toward heat exhaustion.According to the Centers for Disease Control and Prevention, heavy sweating, weakness, dizziness, headache, nausea, and cold, pale, or clammy skin can indicate heat exhaustion. Confusion, loss of consciousness, or a very high body temperature can signal heat stroke, which is a medical emergency. Those symptoms are reasons to stop running immediately and get help—not evidence that you need to become tougher.It’s extra important to pay attention to patterns when running in the heat. Overdoing one workout can happen, but when you string together three, four, or five miserable sessions and begin to dread training, Evans advises you may be heading toward burnout or an overuse injury.Slow down, shorten runs if you need to, or move a session to a cooler time of day. If conditions outside are unsafe—like the sweltering 110-degree summer afternoons common in Evans’ hometown of Phoenix—that’s when you find yourself on a treadmill.In general, Hamilton also recommends carrying fluid on any run longer than about 45 minutes, sipping every 15 minutes or so.Expert Advice for Trusting Your Summer TrainingTo put more trust in your training, evaluate each run according to its intended purpose. Are your easy runs actually easy? Did your speed intervals reach the appropriate effort level? Can you recover and function normally before your next important workout?Next, make sure you actually emphasize running outdoors. It’s easy to swap an outdoor easy run for a temperature-controlled effort on the treadmill, but Hamilton says getting as much outdoor exposure as you possibly can—within safety guidelines—promotes efficient acclimation.If you want (or need) to limit your outdoor exposure throughout the week, Evans suggests keeping the most race-specific efforts outdoors, when you can. Marathon runners should try to keep longer efforts outdoors while those with 5K ambitions will benefit more from doing higher-intensity work outside.Finally, ignore the comparison trap. Judging a humid August easy run against a crisp spring one—or against someone else’s Strava pace—overlooks the actual work you’re putting in when it’s hot outside.“Your heart rate rises, your RPE increases, your pace naturally drops, but none of that means you’re losing fitness,” Evans says.Matt Rudisill is an Associate Service Editor who has been with Runner's World since 2025. A Nittany Lion through-and-through, Matt graduated from Penn State in 2022 with a degree in journalism and worked in communications for the university's athletic department for three years as the main contact and photographer for its nationally-ranked cross country and track & field teams. Matt was also heavily involved in communications efforts for Penn State football, men's basketball, and women's gymnastics. In his role with Runner's World, Matt has interviewed Olympians, world champions, and countless experts in the field to create service content that helps runners of all ages and experience levels train smarter and race faster. When he’s not out jogging, Matt can be found tweeting bad takes about the Phillies or watching movies.