Welp, like has already been said, hike. Find a nice long hill and just walk it with a pack. Find a decent trail or just walk around the neighborhood every morning. However, I tend to think along the lines of that's just going to get me "good enough."

So for me, and this is my own opinion based on my own experience, I do weights and biking.

Not just legs but really all around on the weight lifting aspect. If you're body is well balanced, you're going to do everything a lot more efficiently I've com to find. So don't just work legs, but work core muscles and shoulders and the like. And nothing ridiculous of course. Keep it heavy but not to the point of you turning purple. I could go into more detail but I'll just keep it quick on that end right now.

And I always throw the biking thing out there. I can't even praise that enough. Since I started doing that on a regular basis over a year ago, I've never had any issues with climbing hills or being sore the next day. Putting it this way, I live at just over 2,500 ft. and that's of course what I workout at, but even when I go up to over 9k feet I still obliterate climbs w/o stoppingor getting tired. It's something I've credited to the bike given it didn't become so easy until I started biking regularly.

I always add those extra things onto the hiking part because it isn't always so easy for someone to go take a few hours to go walk, plus all of it adds to your ability. Just my opinion though.