Interesting question.
I competed through HS, college and until my early 30s. I was never a high mileage runner, 70mpw was the usual from freshman year of college to 30. Ran some ok times off that.
In my mid-20s I started to think about when I'd stop racing. I didn't want to be that old guy at road races limping around running way slower than I did in the past. Nothing wrong with that, and I've known a few of those guys, but I wanted running to be something healthy in my life, not an obsession that held me back or slowly broke me down.
I decided that I'd stop competing when injuries started to prevent me from training the way I wanted. In my early 30s I had two serious calf injuries. Both times I was very fit, but those injuries cost me two fall seasons. Both happened on normal training runs. I got in good shape again, and started skipping workouts, long runs, missed races. Gradually stopped training hard because I didn't feel like it, then decided to step back. Final race was a 15:30 5k. Last time I broke 5 for the mile was a 4:50 for the last mile of that race.
Late 30s now, and 40-50mpw work really well for me, closer to 40mpw usually. I also hit the gym to lift twice a week. I run every day with a day off every two weeks. Pace of my runs is anywhere from sub-6 to over 7 and is totally dependent on how I feel. I'll do some strides occasionally, and hit the hills hard on occasion. Feel good and strong most days. That's it. I plan on keeping this up as long as I keep feeling good doing it.