Can you go out longer than 72 hours with a pack? Yes. But those packs are very heavy, or you are eating spirullena (sp, those algea pills) or maybe mainstays, and you have no real water reserve. I mean, maybe a paratrooper wouldnt' complain, but that's about it. (And even then 50 or so pounds of that is usually parachute, and they usually drop a lot of stuff at thier rally point.)

Realistically, after 72 hours, you need to be dug in. It's just a mater of practicality. Even a BIG pack is only going to hold you for 120-140 hours, and you can't even begin to pack that much water.

If I had an idea what precisely you had in mind, I could give you an answer that more in keeping with what you probably had in mind, but before you tell us more, write down everything you need for say, 120 hours. Don't plan on being able to get water that is potable for the first 48 hours. Add up the weight, and the bulk. And ask yourself if you can even pick it up, much less hump it for 12-16 hours at a stretch. If you can, you are cutting a corner someplace or have access to some seriously level after next tech.