This thread is really a variation on the usual "how potentially fast are non-runners" theme. It's unknowable, but the only reference I have is early (age 12-13)school xc races where the whole middle school had to run. Only about 10 kids (the team) actually trained for running. Out of about 85 runners over about 4 miles I finished 16th one year and 6th the next. I did not train at that time, but took up running soon after and topped out my potential at 15:40 having a career that included several years of sometimes regular 60-80 mile weeks. Taking lots of liberties, this hints that if the whole class had trained like me or harder, maybe 15-20% might have broken 16.