I'll be 54 this month so may not qualify as an old hiker .

There was a trail in the NC/TN area that I hiked when I was 19 or so. I was in the army at the time so generally fit, but most 19-20 year olds are probably generally fit. The guidebook said the trail was rated to take 6 hours. I did it in 2 without really trying.
I did the same trail when I was 32. I took the full 6 hours.

I follow several hikers on Facebook including Liz Thomas, former AT FKT holder. She was recently talking about urban hiking. It occurred to me that I've been urban hiking for years. The nice part is the day ends at a hotel with a shower and a meal I didn't have to prepare.

I have always enjoyed hiking in the woods and continue to do so. I also enjoy hiking through cities and towns and museums and other places. I enjoy walking by and looking at old buildings.

So what do I do? I hike as I enjoy and don't worry about it. I accept I'm not as fit as I once was. I walk and exercise daily to maintain fitness, both for health and also because daily walking maintains the fitness I need to do occasional longer walks, whether in the woods, on the beach, in a museum, or through an old neighborhood.

Maybe it helps that I live on a couple of acres near the end of a dead end street, mostly covered in trees (with the exception of where my garden is) and a stream in the back.