I never broke 30 mins, though I feel I might have had the ability.
Last autumn of HS: 19:05 5k road (after 3 years of 30-40mpw)
Spring 1st yr colleage: 16:40, same time dead both road and track (60-70mpw).
Summer 3rd year: 15:34 5k tactical, 8:53 3k. Still miles off a sub 30 10k. My last track season (80-95mpw).
Fast forward 3 years or so where I ran a LOT of cross but due to injuries and heavy academic schedule in spring/summer no track -> was in far, far better shape (100-115mpw).
Career races, finished high up two high calibre xc races 2 weeks apart - in one I was between a 2:17 marathoner and a guy who had been a junior international and brought his 10k down to sub 29 either the following year or the year after. 6 positions behind me, a team mate who had never beaten me in a race (out of ~15 we ran together) - now a 30:22/2:22 guy, still improving. 11 positions back, another team mate, now 30:15/65:26.
Next week, "career"/hobby-ending ankle injury. So it goes.
Never broke 10k, and it's MUCH easier to talk about it than do it, but at the time I was strongly anticipating doing it within a year based on my winter training sessions. One more data point, discard if you will given I never actually did it.