Retirement may be a decision not to train at the level necessary for lifetime performance improvement. Not that anyone noticed but I "retired" at age 30 when I realized that I could improve to a 2:15 marathon from 2:19 only by running 140 mile weeks, but by 1980 I wasn't even in the top three in my club, GBTC, let alone about to make an Olympic team and I had a full-time job, but I could still run 70 miles a week and run in races and work full time and have a life. I didn't like running 10 before work, 10 after, eat, sleep repeat. Five would be OK. So maybe that is what "retire" for a runner means.
Tom