I’ve been in various backpacking clubs (“overnighters”) and the number of older hikers decreases with age.  There’s physical health but also I’ve noticed even mental health can affect older hikers who have had excellent physical conditioning.
Big thing beyond general health is take care of skeletal muscle, as your cartilage, etc.. cannot really repair itself quickly regardless of age (it’s a blood supply thing).  Here’s a reviewed article on leg exercises from an older hiker (age 81) now a trainer, who was told he had “too thin” cartilage over a quarter century ago.  Muscle tone also helps for physical therapy after getting knee replacement surgery.
https://www.wellandgood.com/exercises-arthritic-knees/
I started backpacking pretty late (mid 30s) and a little over 20 years later I’m trying to get the bucket list of longer hikes (or bikes) done.  In my 60s, my plan is to hike the “state long” long trails.  I had a desk job as my last temporary assignment before earlier retirement, .. and that wasn’t very good for my health.  By training about everyday (even if only a couple miles of brisk walks/runs), I’ve been able to improve my “daily mileage” on an ultralight trips, though I’m not usually fast (see below).
If I need to lose weight, I’ll walk/jog a bit more pre-dawn before any calories .. just a cup of water.  Now I have 1 “cheat day” diet-wise but that’s also weight lifting day.  My slightly younger brother facing a weight/cholesterol problem was told by a doctor ..
.. if it’s delicious it’s not for you ..
but also think people need one day to enjoy before returning to broccoli/kale salad with air fried tofu “surprise” the other 6 days of the week. Also there’s a limit to dieting as too much calorie restriction can lead to not getting enough nutrients.  Then there’s genetics which are beyond the realm here, but think trying to be “healthy” as possible is enjoyable in of itself (and broccoli can be made better w/a pinch of salt .. almost as delicious as bacon once your taste buds are trained).
Time: I can now hike a mid 20 under “UL conditions” but the slower the better.
Temperature: 3-season.  I actually like snow and winter, but didn’t like my hot coffee icing over.
Terrain: Everywhere I go, usually there’s a rule to follow the trail network due to LNT guidelines.  Not that I mind getting off trail if there’s some scenery.
Goals: keep on keeping on. Probably add more stretching and even yoga.
My family wonders why not just sit on front of one of those Peloton machines, sweating to the music? Â Maybe the views?

(Trip from a couple months ago .. Crater Lake OR even though a fall deng near took out my knee a few days prior [shoelace snag] .. next day the wildfire smoke came in so you gotta hike it fast in late summer).
Just to add another thing is not comparing oneself to younger hikers. Â They can haul heavy loads for many miles daily with enough trail miles.