It's certainly the case that a specific training program might maximize the talent of a runner in high school. And completely possible that in my own case I got everything out of my body with 2.5 years of training 6 months a year, only doing high speed intervals and low mileage, basically rounding into shape the first 6 weeks of track and dropping big times for a few weeks at the end of the season. But most evidence shows that endurance is very trainable and that it takes years to build it up. If that's not the case, why do all of our pros have their peak performances in their mid to late 20s?
You and I may be the extreme outliers. Most people will improve with a better endurance base. In my own case, I went to college, blasted everything 110% and overtrained myself into oblivion. I ran worse. It wasn't really increased mileage, just too many sub 6:00 pace "recovery" days. I do think I'd have been a lot better with a properly designed training program rather than the "keep up with the fast guys you pu$sy" program that was my college experience.
I had two other teammates in high school who were within seconds of my PRs who went on to huge success in college and beyond. One made a world championships marathon team and the other had solid college times for the era (4:06 mile, 8:13 3k, 13:59 5k).