Not like you really wanted or needed more, but another comment on the physical stuff...

I totally agree on the importance of the core stuff, but if the goal is to build functional endurance on rough terrain, one of the most important (and overlooked) aspects is building agility and balance. Best way to do it is to do it: off-trail scrambling in mountains, boulder-hopping up river beds, whatever you can get. If you don't have access to terrain, take up sports like soccer, tennis, anything with irregular, unpredictable movement.

In my neck of the backwoods we get guys all the time who are extremely fit by objective measures, but who burn out in no time. Because they got their fitness working out in a gym and running on tracks and roads, where the movements and terrain are regular and predictable, they have a terrible time adjusting to moving over irregular ground.

Agility and balance give you efficiency: you use less energy to traverse a given piece of ground, and the difference is dramatic, not marginal. Again, best way to do it is get out in the bush, off the trail, and explore. Good fun, channel the inner hillbilly and go for it. Bring the fishin' rod and the dawg...