The distance is the identity. Somebody preparing for a first ten kilometres and somebody chasing a marathon time describe themselves by the number, and pages built around those goals match how people actually search.
Beginners search full of doubt. Whether they are too slow, too heavy or too old to start is typed constantly, and a club that answers those honestly gets the members who stay longest once they arrive.
Pain brings runners to coaching. Shin splints, knee pain and returning after a layoff bring runners who have been training alone and have hit a wall they cannot explain, which is exactly the problem a coach solves.
Race names are seasonal traffic. Everybody entering the local half marathon looks it up, and a page about preparing for that specific event reaches a large, motivated and entirely local audience.