You certainly don't need that much water and it could well be harmful to drink so much. Just drink by thirst and don't plan to drink a certain amount. If you find yourself drinking too much out of habit you may need to have smaller amounts at a time, then waiting 30-60 minutes before having more if you are still thirsty then.
2 gallons of water gives me visions of the X-Men movie!
I'd say you have plenty of body fat to survive for three weeks. Just listen to your body, it will tell you if it needs food.