Baby, it's hot outside — but what's a devoted runner to do? "Match your workout to the weather by slowing down during exercise and seeking shade afterward," says Samuel N. Cheuvront, PhD, a research physiologist at the U.S. Army Research Institute of Environmental Medicine. To beat the heat, heed his 411. Pick sunrise or sunset. Your best bet on a hot day is to head out in the early morning or evening, when your shadow is twice as long as you are tall. According to the Nation