Definitely the training. I was in high school in 1998-2002, so not too long ago in the big scheme of things. I was a low to mid 16 minute XC runner at a small school, and I trained with one of the top big school teams in the state because no one else on my team was even close to my ability. We definitely trained way too hard, as there was no concept of heart rate training or controlled efforts.
A typical non race week in the late summer/early fall looked something like this for me:
Mon: Hard 4 mile tempo 5:30ish pace (pretty much a time trial effort).
Tuesday: Base mileage, 7-9 miles, ran at 6:30-7:00 minute pace
Wed: Hill repeats 8x90 seconds (pretty much all out.)
Thursday: Base mileage, 7-9 miles, ran at 6:30-7:00 minute pace
Friday: Longer intervals, such as 6x1k at 5k pace (pretty much all out)
Saturday: Long run 10-12 miles, descending to tempo effort
Sunday: easy run
Obviously way too much intensity for a 16 minute 5k runner in high school, and the easy and long runs were wayyy too fast. If I was coaching myself today, I would slow the tempo run down by 20 seconds per mile, slow the base miles down by 1 minute per mile, change the hills to a shorter length, ie 30 seconds or so, and do the long run at a "zone 2" effort" and descend the 6x1k so that I am not digging myself a big hole by doing each one at 100% effort. I am pretty sure I could have ran much faster in high school, as once I started training properly in college I was able to get into the 14s pretty quickly.