GREAT QUESTIONS! And ones I've asked myself and others almost weekly these days...
I'm 51 years old and have been running for 30 years. Amazingly, I ran all my PRs after 18 years of running, when I was 39 years old. Nothing great, but not too bad (5k = 16:04, 10k = 33:51, 10 miles = 56:35, HM = 1:14:17, MAR = 2:43:19).
During that time, I was feeling pretty good about things when a friend said to me, 'just wait until you hit 45, then it all starts going downhill'. I swear he put a curse on me because sure enough, right around that time, my race times started to fall off. Now that I've crossed into my 50s and dealt with some long-lasting injuries, I'm struggling to run track repeats in times I used to average for 5k!!!
Don't get me wrong, I still love to run and do workout but it is tough to get motivated to race when you know you'll never approach times and places you once did. Racing has become much more of a social outlet to me now, as opposed to a quest for the next PR or trophy.
I do agree with finding different distances or events to try, as well as age-grading your race times to see if you can stay in the same ballpark as when you were at your peak. So far, I've managed to age-grade run the equivalent or better of my lifetime PRs, so that's good.
I also find as I get older that I'm imparting my running "wisdom" more often to younger, newer runners. Whether they want it or not. :)
And I always make sure to live by this saying:
THE OLDER I GET, THE BETTER I WAS. :) :) :)