I would suggest that by far the most critical aspect of maintaining fitness comes about through a change in attitude. A person needs to recognise that the body won't always be there when they need it unless they incorporate a regular maintenance regime. As the chronology mounts up "Use it or lose it" must become an unspoken mantra.
Up to the mid thirties, for most their body will fulfill any demands placed upon it. During the next twenty years, there is a gradual deterioration, often ignored. By mid to late sixties, bits start to fail unless adequately maintained.
Whilst the medical professions can "patch up" worn bits, the repair will never work as well as the original. So ongoing maintenance becomes a daily neccessity, with a need to keep pushing the envelope, because once relaxed it becomes incredibly hard to retrieve the outer limits.
As a regular golfer I never cease to be amazed by the number who choose to drive around the courses in carts rather than walk. At 67 plus, I never ride. If I can't walk the course then I would see very little to be gained from the time spent, believing that just swinging a club isn't much better than playing a game on a computer screen.