The #1 example IMO of a lousy college runner who made good is Ron Daws, who ran at the U of Minnesota in the late 1950s but wasn't even good enough to travel with the team to away meets. In fact he quit the track team his senior year IIRC.
Daws's college bests were 4:30 for 1 mile, 9:43 for 2 miles, and 15:22 for 3 miles (not even 5K distance).
Daws graduates, decides to keep running because he loves it, but has no coach and no support. He gets married and works full-time, and runs in his spare time. During work breaks he runs on the building fire escape.
In 1964, at age 27, he finds a book by Arthur Lydiard, studies it, and ups his mileage to 100 mpw.
He can't find good running shoes so he makes his own shoes. He has no money so he builds a treadmill from spare parts.
At age 28 he sets American track records for 15 miles and 25K. (OK, odd records, but he did have them.)
In 1968 at age 31 Daws qualifies for the U.S. Olympic team by finishing 3rd in the marathon trials, in brutal heat at Alamosa, New Mexico, ahead of a slew of runners with better PRs. At Mexico City, Daws finishes 22nd out of 85 runners in the marathon. To him, that was the high point of his running career.
He also finished in the top 10 at Boston four times (4th, 5th, 9th, and 10th).
If you can find it, read his inspirational but scarce autobiography, "The Self-Made Olympian."