A local road race 5k might have a thousand entrants. You might feel bad about occasionally walking and finishing in 500th place, but that's just in the subset of the population that participates in those events. The biggest race in our 1,000,000-person metro area draws about 20,000 entrants (many from outside that area), so the 50th percentile in the race is 99th percentile in the population.
Similarly, there's a big difference between people who participate and people who practice. Every high-school cross-country team will likely have several kids who can run under 18 minutes in the 5k, just because they practice 5 or 6 days a week. Elites peak at about age 28-30 in the 5k, so these 16-year-olds aren't as fast as they could be, but they will likely never run faster than they do in high school because they won't practice as effectively as adults as they do when coached on a school team.