over an hour period I don't think it will be very hard to do unless you have a really small stomach or appetite. I frequently consume close to 1/2 gallon of water, milk, gatorade, etc when I am really thirsty or hungry at one time. So I would have no problem consuming another 1/2 gallon over an additional hour. Of course I am probably a pig.
Just drink as much as you comfortably can at first when you start, then every 15 min or so do the same, you shouldn't have a problem.