If you want to make 2 gallons last for 72 hours, then you must also carry food that does not need to be reconstituted. There won't be enough water to be able to cook anything.

Oh, and when you wash out your pot you should drink the wash water rather than tossing it aside. Oh, and I would not plan on covering my maximum mileage each of those three days; I'd leave myself some safety cushion. To be honest, though, I'd never try to make 2 gallons of water last me for 72 hours in the desert.