Here's a question from Runners World this month: During a race, should I drink at every water station?
The answer is; it depends on your weight. They suggest testing how much you need by going on a 60 minute run. Weigh yourself before and after. The amount of weight lost equals how much fluid you typically need per hour.
So, I figured I'd give the test a shot. I got up this morning, ate a bit, hopped on the scale and logged 191 pounds. Cool. Then, I ran for exactly 60 minutes. Now, it's getting warm in Kansas City, but it's not too bad in the morning. It was 71 when I left the house and almost 80 when I got back home.
I took off my soggy clothes, hopped on the scale...185. Geez. 6 pounds in an hour? I should write a book.
If you assume 1 lb = 16 fl oz (it's actually 1 oz = 0.96 fl oz, but whatever) you'll discover that I managed to sweat off 96 oz of water. That's 3/4 of a gallon. That's a lot of water.
No way I can do that much water per hour at a race.