Personally, I find racing motivates me to keep fit. I love competition, and I enjoyed structured training.
I'm 65, and think I'm in about 41:00 min shape (ran a 5.9 mile time trial last year at 6:35 average). I thought that was pretty solid, until I saw Alastair Walker's phenomenal effort.
I'm now further away from the world-record than I was when I was 25 years old.
I think there are a handful of masters runners, that are just genetic anomalies in terms of both having the initial ability, and then being less affected by age related decline, primarily I suspect in terms of being able to recover relatively quickly. If I do a decent session, it then takes me a couple of days of shuffling around three or four miles in the 8:00 min mile range before I'm ready for another.
Guys like Tommy Hughes and I suspect Alastair Walker, are able to maintain higher training levels, which among other things helps keep the weight off.