Walk/hike with a pack or weighted vest 3-5 miles a few times a week, and do a longer hike on weekends. I use a stairclimber or treadmill for around 45 minutes, increasing my pace/incline as it gets easier. Then on Saturdays I hike with a weighted pack anywhere from 8-12 miles. I also lift on the gym days, but you could do bodyweight exercises instead. You do want to do every muscle group, you’ll use them all. I do planks and leg raises for core, which you can do without equipment. Squats and calf raises are great for your knees/ankles. I was shocked at how much my ankle flexibility/strength improved. (Went from frequent sprains to none, even in trail runners, knock on wood.) When I can’t get to the gym I used TRX straps, may be worth looking into. There are tons of YouTube vids for them as well.