Sometimes you just fade away...
I realized I wasn't able or willing to train as much or as hard as I needed to just maintain where I was. As I got older more training was needed to surpass where I was and get better. I never really had any serious injuries. After years of running 100 miles a week you just get bored.
High School: 70-80 miles a week at peak, 50-60 average.
15:53 5k, 9:32 2 mile, 4:27 mile
20s: Average 90-100 miles a week, peaks of 120-140.
15:47 5k, 1:09 half marathon, 2:32 marathon.
Stopped running competitively in 2007 right before my 30th birthday..:)
I've run 9 or 10 marathon, 5 or 6 half-marathons, 5 or 6 10 milers, hundreds of other races. I've won a lot. Won some gear. Won some money. It was fun, but then stopped being fun and started feeling like a burden.
So....I started lifting weights, put on some muscle, got ripped, won a sword at a bodybuilding competition, joined the Army and now run and lift because I enjoy it.
I'll be 37 in January and enjoy working out more now than I did 10 years ago. I run 30-40 miles a week, about 5 days a week, am 5'6' 170lbs (vs 140 as a competitive runner), still lift 3-4 days week, and still have always been the fastest runner in my unit running "only" 11:00 2 mile. On a post of over 20,000 soldiers I still place top 3 at any post wide competitive 5k.
Find pride and enjoyment in whatever little circle you want to fit into. I enjoy being able to outlift the few faster runners I've come across and being able to outrun the stronger lifters AND doing this at 37.
It's very interesting. What is deemed "fast" in the competitive running community and what is deemed "strong" in the serious lifting community is vastly different that what you could realistically say is "fast" or "strong". Go to any local 5k and people who train and run for these events struggle to break 18:00. I'd guess the top 1% will break 18:00. Go to the gym and watch people lift. See who is really strong. Watch hundreds of people lift weights, try to get better, try to get stronger, maybe 1% are benching over 225 or deadlifting over 300.
Anyway. The key to athletic happiness while getting older is enjoy the little things, enjoy the little victories, maintaining is a victory..:)