I love seeing people getting into bushwalking, at any age. I knew someone who didn't take it up till she was 67, and did the most amazing multi-day walks! And I have other friends in their 70s/80s who have still been able to do 6 day walks.

So don't limit yourself too much just yet.
I also found (till I sprained my ankle recently

) that I was able to get fitter' than I've ever been before, even though i'm up over the hill and heading down the other side. That came through the opportunity to do lots of walks - probably helped at one point by going to the gym, but really, I just want to be out in the bush.
Oh, and another significant factor for me was reducing the weight of my pack by over a third, with lighter weight gear, and deciding I could leave some things at home. I almost floated up to Marion's Lookout last time!