I'm in my 50's, and on the downhill slide after setting all my PR's in the early 40's.
I regret that I hung up my track shoes at age 18, when I could have developed into a decent D3-type runner. Didn't run again until 38 YO, and was able to win half a dozen open races (smallish fields) within a year or two.
Anyway, I think "What could I have done with my performance level if I sought out a good running school or coach?"
What would you change?