ran 4:25 mile and 1:59 800 as a high school junior off 3 months of solid 35-40 mpw training. got injured a bunch from having bad form and from over-training after that and didn't pr again. now i'm 29 and my fiance is a PT and we figured out what muscle groups to work on to keep me from getting injured. i'm also way more mature now and am very patient with increasing mileage (back as an 18-20 yr old i'd just more than 10 mpw at a time which was dumb). i just did my first 30 mile week in a decade and i'm feeling great. ran 4:56 off no training a few months ago, ran 4:45 after a month of 15 mpw training, and just this week ran 4:36 off 20-30 mpw training (2 workouts a week).
if i get my mileage up to 40 mpw without getting hurt, and increase the quality (harder workouts, maybe 3 hard days per week), and keep it going for many many months, perhaps even years, is PR'ing as a 29, 30, 31 year old in the cards for me? could i possibly go under 4:20, which was always my goal in high school that i never got to do??