I'm surprised that no one mentioned heartrates during this conversation. During my 25 mile ride home from manhattan on the 102 deg day last week, I noticed that my heart rate was steadily climbing. I usually have to incorporate sprints to keep my rate at around 140 for the hour and a half ride, but in the heat my rate steadily climbed until at around 15 miles it was at 170 on a flat and I was not pedaling hard. I stopped and it didn't go down so I hit a starbucks for an iced tea and the AC and watched it steadily drop. The heat is a serious consideration even for those who are in decent shape.