Admittedly I have not run a legit track race in many years. But time trialed a 4:58 in the full mile solo last spring at a "race weight" of ~215. Much slower than my college PR, but same as my HS 1600 PR at 65 lbs heavier. Given that I was exclusively focused on running in college, even though I'm slower, I think I'm in much better overall shape now.
Diminishing returns is definitely real, but if you think about it, the gains from not finishing without to stopping to a 20min 5k in 2 years and then 20min->18min in another 2years is actually a good amount. Never ran the 5k in college, but I was definitely in under 17 shape (but probably not under 16:30; I always was better in mid D events).
I'm not going to hit PR's anymore (unless you count weight class or age group PRs), but knock on wood, I think I will still see improvements in my overall fitness for years to come. Slower progress, but progress nonetheless.