I also disagree with Yury that hydration is over-rated. He may be the only person I've ever heard make that comment. It's essential to hydrate, and most beginners will never drink too much. I've been hiking for many years, and don't know anyone, nor do any of my hiking friends, of someone who overhydrated. Mostly it's a myth - drink more fluid than your body needs and you'll simply pee more often.
Kevin, hyponatremia is not a myth, though I've never heard of a hiker having the problem. I have seen write-ups describing novice marathon participants suffering from it. The most famous one, though, is the Sacramento, Ca, radio station that held a "Hold your wee for a Wii" contest. One woman (who lost) went home and died due to the problem. (Her survivors won much more than a Wii.)
