I can hear it now "here comes the village idiot"
Rowing. The intensity dictates what you get out of it, and time spent can increase pure endurance. It's a total body workout that doesn't create excessive impact on joints, but does increase their resiliency. Since altitude and large hills aren't available easily to me without a significant time sink, I've found this is far easier to facilitate and has made my higher altitude efforts much easier. Because of this, the weighted ruck hikes I do once a week (at best) on flat ground get too easy and I have to add weight or hike longer every time. It's supported every trip I've made to hubt at elevations exceeding 8k ft, up to around 11k.