I have done it too. Its really hard. But we had only a half hour to do so. I didnt finish, but a friend of mine did. Didnt go to school for a week after.

I think i read somewhere that the stomach cant hold that much liquid. Anyways, i would give it a go and pace yourself. An hour is alot. I finished most of the gallon when i did it in the half. YOu should be fine. I also weighted 160 when i did it so you can gauge yourself.