When I did the Disney World Marathon I was feeling great. So great that I decided to skip water stations. I’ve done it before in 10ks and half marathons, right? What a mistake. I started cramping around mile 18 with just twitches and by the time I got to 21 my calves cramped up so bad that it locked my legs up and it made me go to the ground because I couldn’t move them at all! Here’s a very simple article by Carmichael Training Systems on hydration.