I do have the advantage of living at altitude (~4,850' at the casa), so that helps. I try to stay in reasonable shape, but as I approach 66 this summer, there are a few aches, pains and "life events" that have slowed me down just a little.
Funny story on that - a few short years back I was doing one of the 315 trips on the C&TS and one of the photo stops (well, several, but this one in particular...) involved a scramble up some rocks. When we got back on the train, a woman sitting across from me started griping that such locations were all well and good for "youngsters like you". After a quick round of "just how old do you think I am?" I discovered she was a good 15 years my junior.
Scott