"There's no way I can carry 3 weeks of food so presumably there will be a point where I'll run out if my journey takes me longer than 72 hours (or however much food I'd planned for)."

When you head out with 3 days of food, one hopes that you will be able to supplement that with local resources such as berries, nuts, small game, relatives.... thus extending your carried food.

One thing to be aware of is always to keep an eye out for old "homesteads", burnt out buildings, concrete foundations, etc. Many old gardens had edible plants that have reseeded themselves with something useful. I once came across an old chimney, and while prowling around, came across what looked like a potato patch. Digging a bit, I found a source of spuds!

When people leave "stuff" behind, some of it may be in their gardens.

Sue