If you never have to worry about actually performing at the top of your game in the heat, then by all means head inside to the treadmill, says Chris Jordan, creator of the 7-Minute Workout and director of exercise physiology at Johnson & Johnson's Human Performance Institute. But if you're training specifically for a race in late summer you should be doing the majority of your runs in similar conditions.
There are, of course, pros and cons to both types of workouts. "In the air conditioning you'll have less heat-induced stress on your body, which means you can devote more of your cardiovascular capacity to running as fast as you can," says Jordan. It can also serve as a much-needed reprieve on days that are particularly muggy, or when your only time to workout is mid-day under the blazing sun.
Running in the heat, on the other hand, puts more stress on your body—and you'll likely notice, at least at first, that you can't go as far or as fast as you would be able to in cooler conditions without expending more effort.
"The cardiovascular system has to work extra hard, not only to pump oxygen to the muscles you're working but also to pump blood to the skin, for example, to help dissipate heat." Humid conditions make it even more difficult for the body to regulate temperature, he adds, since sweat doesn't evaporate off of your skin (and therefore cool it down) as quickly.
The good news is that you can adapt to the summer heat, as long as you give yourself adequate time and training to do so. But spend a few times each week outdoors and your body will become more efficient at sweating (bad for social situations, but good for training) and at sending blood to the skin, without compromising blood flow to the muscles. "It's like any exercise program; you've got to do it regularly and progressively."
Bottom line: Take precautions when you run outside—wear light colored, wicking clothing; avoid peak mid-day heat; hydrate before, during, and after your run; and stop if you experience signs of heat-related illness—but save the treadmill for only the hottest and most humid of days. Your body (and your race time) will thank you in August!