Sit and kick
Personally, sub 20 is a fair marker I think, maybe sub 21 in the later 40s
There's a world of difference in the never-ran average Joe's that get off the couch on their 40th birthday and ex collegiate runners
One example might be in swimming, I'm an ex collegiate sprinter, I took 20 years off and entered a 50 for grins, I still got under 24s which isn't amazing (20.n in college), but there is literally no adult onset swimmer that would come within 5 body lengths of that, regardless.
Sure sprinting isn't distance, but a similar concept. I took the sport to a high level in college and easily trounce my AG in masters meets. Running isn't terribly different, unfortunately we are all a bit differently able.
Disclosure: after 4 years of ~15 mpw jogging,, my PR is 20'14 in a Turkey trot lol
Gobble gobble mfer