I am starting to wonder - is my mind younger than what my body can handle? Is it reasonable for me to expect to get in a good 10-20 years covering some places like Glacier, Alaska and others? Or am I dilusional?
Nope. you're not. I'm younger than you (42) - but you're probably in better shape. I'm a fat computer nerd with a streak of stubbornness and determination and a long history of crawling around the bush.
You better have a good 10-20 years in you because I intend to.
Once you're *comfortable* outside, really, it's just walking. if you're not trying to run a marathon, and you don't mind getting up with the dawn, you can do some very impressive hikes and some very nice places at your own pace, and enjoy them immensely.
I like dragging fat out of shape nerds into the bush. once they figure out they're not going to starve, or be cold, or die in what I'm having them take, and it's just walking - they're hooked.
With any kind of modicum of conditioning (which it sounds like you have) you'll do just fine! just get out there!