You ARE a young fellow! At your age, I got back into alpine climbing after years of raising kids and going to graduate school (to get a good job so I could pay for those kids college, LOL). I then quit climbing at about 65, not because I could not do it, but because all my climbing partners "aged out". I now backpack because I can do it solo. Now, a few weeks from 69, my summer plans are a 300+ mile off-trail thru-route in the Wind Rivers.
Two main differences from ten years ago; pack weight and maximum distance in a day. I limit each leg to 10 days, and take a few days off between. I am not "UL", but my starting pack weight is about 30 pounds (including a bear cannister) (20 pound base weight on my back) for a 10-day trip. I go light on food becaue I fish a lot. I usually do 7-8 mile days off-trail or 10-15 miles on-trail. I did a 21 mile, 11 hour day (with 3000 feet gain) two years ago, and it really did me in!
I also have to limit my downhill to about 4000 feet loss, or my knees get painful. I can still go up 5000 feet in a day. I also have to be willing to just stop and camp if I feel my body is getting over-worked. Some days are just "off" days.
My theory is that as I age, I keep in shape off-season by doing more gentle excrcise - mostly walking the dog 3-5 miles a day and 20-mile bicycle rides. No more running for me. My friends and family think I am obsessed, but I am VERY careful to keep my weight down. Never want to get more than 5 pounds over my ideal weight. I am also lucky to live where I can backpack in the winter, although more overnight stuff.
My eyesight is one of my worst health problems. I compensate by making sure all my gear is bright colored, otherwise I loose it! Also need bifocals, which makes boulder hopping a slower process than when younger.
I am going to get an InReach this summer, as I realize that I am getting more likely to get injured.