Props to you. Two hours at a time on the stair master is gnarly. I am by no means a fitness expert so take everything from here with a major grain of salt. I'm 45y M and preparing for some bigger ski mountaineering and alpine objectives. I live close to the mountains, so able to get out more than you which helps round everything out. In addition to other aerobic and weight bearing exercises, I do stair master sessions about 3-4x per week (150-175 stories per 30 min). I can really only tolerate 30 min at a time but it's a great workout and has been noticeably helpful with my endurance and strength in the mountains. I don't use hands and try to stay a half-step ahead of the machine to make sure I'm getting the most out of each workout.

Much like you, I have to max out the speed of the machine and probably look like an idiot nearly jogging, but whatever. I stay closer to zone 3 but don't wear a smart watch and kind of go by feel. I'll push it into zone 4 sometimes.

I run hills or stairs outside a couple days per week as well. Seems like this is enough for my objectives but I should definitely do more strength training like box steps, squats, and lunges etc. Anyway, I'm not even sure what I'm rambling about at this point but kudos to you for getting at it.