I started when I was 16 back in 1985. Before that, I had run the big local 10K two or three times without training, running 58:xx, and 56:xx (and maybe 54:xx?). In the Presidential Fitness Test as a freshman or sophomore, I ran just short of a mile for the 6-minute run on a dirt track - maybe 6:01 or 6:02 for the mile. I was a road cyclist since junior high, riding more than an hour per day, usually on a route that had a steep 1.7 mile, 775 ft climb, before starting running.
I started out running 9 miles a day (8 miles the first day) starting out, just doing what I could to get through my route (slow slog), which was mostly the same every day. It was summer, so I didn't have coaching or hard workouts, though I had some occasional get togethers with the high school cross country team that I was about to join . Three months later, I was running varsity on the cross country team, and was 2nd runner for the team in my first race. We didn't do 5Ks in cross country then. Mostly they were 2.83 miles for the course that we used for the dual meets/league finals. Started out 16:30, then 16:00 and then maybe under because I remember sub-5:30 pace, though that might have been a year later. Later in the spring, I ran 4:57 for the mile my first time in a high school track race, but only got down to 4:48 (dirt track mile) before the first year was up. I didn't have the best coaching. I was really killing myself in intervals, and wouldn't feel right for two weeks after a session because I thought that was what I was supposed to do. So was mostly overtrained, with sore lymph nodes, etc. my two years of high school running . With better (easier, more controlled) training, I think I probably would have been around 4:30 the first year. I think I did a 10K that first year (same road race as my previous 10Ks), but it was a easy controlled tempo in 36:xx because it was in middle of track season.