As someone who can run at a pretty competitive pace, speed isn't everything. I notice that operating at the top competitive levels of most sports tends to destroy your body pretty quickly -- lots of stress injuries, sprains, accidents, etc, simply because you're pushing it so hard so often. When I really tried to push it, I could go faster and further... at the expense of actually enjoying myself.

I trail run in a much more relaxed way now -- enough to get a good workout, but definitely not as competitively as I used to road run. I've also picked up trail biking, and it's been SUCH a relief to just not think about my speed. I focus more on the experience of enjoying nature and I feel like that's a much healthier place to be. You can still push it occasionally, but there's a really pleasant niche at 60-80% of your max capacity! I also get the opportunity to share the sport with friends who aren't quite as speedy as me; turns out it's more fun to be social during an activity than to push myself.

And I have to say: at 53, even 4 hours is very very respectable. Try to remember that these things are first and foremost hobbies that are supposed to be fun!