About 15 years ago, I participated in a fund raiser bicycle tour in Michigan, in early July. Temperatures that weekend hit almost 100 degrees, and the humidity was about 90%. I made it through the first day, but got a nice case of heat exhaustion on the second day; I managed to finish the route, but about half of the riders did not.
I want to talk about this past Chicago marathon. I'm sure you remember reading about it; it's the race in which a runner died. You can read a news article about it here:
http://news.yahoo.com/s/ap/20071008/ap_on_sp_ot/run_chicago_marathon
It's a simple, and sad, story. The race was scheduled for early fall, but the weather was unseasonably hot. As the article notes, 10,000 runners did not show up, and another 10,000 didn't finish. And one runner died. At first, it was thought that he died of heatstroke; it turns out that he had a hidden heart condition which caused his death. Either way, I feel sorry for him and his family.
In any case, this past Chicago marathon is a good object lesson for any runner (or cyclist) out there: make sure to pack water. And lots of it. The biggest threat to runners and cyclists is not traffic. It's dehydration. It is absolutely vital to replace the fluids you lose while you exercise! This is even more important on a hot day.
Everyone's metabolism is different, but I have found that, for me, about half to 3/4 liters of water per hour is sufficient. That's about 3 pints. Yes, that does sound like a lot, but remember, you'll be sweating and breathing it out as your body tried to cool itself.
So the rule to remember: When you're thirsty, even slightly, drink!
