I know someone who had no HS or college track experience. Instead of becoming a hobby jogger, he trains for one of the most brutal races in track (the 800) at age 26 or so. Miraculously, he sticks with it for a few years and runs unattached in several low-key college meets. I think his PR was 2:08 or 2:07, and he did all of this without ever having a coach to guide him or teammates to train with.
Anyone know of any other similar cases?