Genetics plays an important part in out physical fitness. YA gotta pick the right parents. Some of us have "glass knees" (or hips) and no amount of yoga or vegan diet will prevent them from wearing out prematurely. So, in this age of wonder medicine, we get new knees or hips and continue on.
But other body parts, like the "main muscle", the heart, and other muscles, tendons, ligaments and lungs, can be helped by lifestyle. I hike with older Sierra Club members, many women in their 70s, who can easily keep up with me at 9,000 ft. and carry on a conversation. (Try it at 9,000 ft. with a 20 lb. pack.) They are amazing Old Girls.
The point is that we can usually "keep on truckin'" as the hippies used to say, if we are consistent in our daily exercise and diet - with time out for a good wine or Most Excellent microbrew.
I read in this thread of many overcoming physical problems and going on to enjoy backpacking. Congratulations on your preserverence. When you stand, pack on your back, at a fantastic vista, you've gotten your reward. When you see inactive friends of your same age decrepit with diabetes, heart ailments and creaking around with obesity, all the time struggling with big medical bills,you know you've lived the best lifestyle. So celebrate and hit the trail.
You've earned your health. It's given to us for "free" in our youth but those of us who truly understand find it must be worked for to keep it the rest of our lives.