Im just making a number up here, but maybe 90% of the general population has no need to maintain any real level of fitness at all. It isnt required for life, or their jobs, or their leisure activities.
So why does anyone focus on fitness at all if they dont engage in competitive pursuits?
Because they compete with themselves. Whether its to see how far they can run, or how fast they can run a marathon, or how much weight they can lift.
Hey, its something to do. And generally, no matter what you do , its a healthful benefit and good way to relieve stress.
Do something, keep moviing, and as you age your quality of life will be much better. For sheer convenience, resistance training and cardio is the staple of fitness workouts. No, your not working fast-twitch fibers and your not going to be an elite athlete. But guess what, you arent anyway, no matter what you do.
As you age joint issues become predominant in many people, particularly back problems. Low impact excersises like elliptical cardio machines beats running, and low weight /high reps becomes necessary for resistance training.
I visit a gym when Im out of town for work that has a seniors program. I am amazed at some of the older people there and how little they can do compared to others their age.
Im a believer in high intensity interval training for cardio. You can get more benefit in a 20 min hitting peak intensity several times, than you can in 1 hr of slow easier effort. If you arent drippiing with sweat and breathing hard, you arent working.
Of course nothing works as well as carrying a pack up mountains, and breathing hard, 8 hrs per day for day after day.