This is key, as exercise is essential to a certain degree. I've crossed off my bucket list 4 marathons within a 12-month period and one half marathon a bit later. Fortunately, my knees and frame did not suffer from that period of time. The amount of training that goes into preparing for a race of that distance is also consequential, as you must establish a base of fitness from gradually building your distances as you get closer to the race. That's a lot of running and a lot of chronic cardio. I was intrigued by ultra marathons after running a few marathons, but family commitments and work travel intervened before I came to my senses. There are much easier ways to stay very fit without the cardio focus and time sacrifices. I think many of us who thought these activities were good for us just blindly got out every day for several miles thinking we'd live and be fit forever. It was the injuries to the heart from overuse and taxing it beyond its capabilities that really got my attention. Just like how we react to different nutritional approaches, some of us can handle more cardio work than others. Today's research shows that smart, targeted exercise is far more effective than the alternatives of either chronic (OCD fueled, that was me) and no exercise. As we get older, focusing on those activities that retain and build muscle mass are far superior to other approaches.

Edit: spelling correction