SmarterFitter

You might be interested in this from Peta Bee at The Guardian:

"We evolved from hunters - we had to run and chase animals on the hot African plains. We didn't have time to pause for a drink," he says. "Physiologists developed an unproven hypothesis that to become even the slightest bit dehydrated during exercise would kill you. The sports drinks industry then used this bad science to market their products." Runners have died from hyponatraemia, but Noakes says he "has yet to find a death from dehydration in the history of competitive running".

The article talks about different rates of dehydration for different people and a bit about how to predict how much water you should drink based on your weight difference before and after running.

Link to full article