It’s well proven that to stand a chance of racing well in the heat, you have to train in the heat. But sauna suffer-fests have another benefit: They prepare you to run faster in cooler weather, too. In 2010, researchers had competitive cyclists spend 10 workouts pedaling at a low intensity in a 104° F room, while another eight duplicated the effort in cool temps. Outside of the lab, everyone maintained their normal training regimens. After two weeks, the performance level of the control group remained the same, but the heat-acclimated group surprised researchers by not only showing gains in hot temperatures but also charting five to seven percent gains in a cool environment.
Hop over to Runners World for another great article about training in the heat. Man, I needed to read that after some unpleasant runs as of late.