Don't freak out. I drink over a gallon of water a day. Plus, other stuff. I have for years. I do it slowly throughout the day. No I'm not a mental case or diabetic.
Just make sure your not skimping on the salt and take your vitamins. Definately don't over do it all at once - especially when you are biking or running hard. It can be fatal.
During the week I will keep this habit up then on the weekends I drink like a normal American - or Southern American (sweet tea, soda, lemon water, coffee, white lightning).