I'm building an aerobic base with running at present, 53 miles last week, and gym based ME. As summer comes on I'll add heavy pack work and heavy pack/steep hill ME. Also, start just getting out with a pack and rifle and trying to kill something legal come spring. The best training load is meat and fur, if only I was any good at getting it.
Proper prgressive "stress" results in the stressed parts becoming stronger. This includes the knees and back. Low flow tissues like tendons, ligaments, and cartilage respond slowly, but do respond. Injury comes from doing too much too fast. Avoiding stress is the road to weakness, stacking on too much is the road to injury. Balance is key. Too many people are scared of it. In the 1960s the consensus was that a human would die if they ran much further than a marathon, now we have 450 mile foot races that are not stage races. With enough time and slow progression, the body is pretty amazing in what it will adapt to.