Myself and two of my HS teammates all ran D1 via different paths. I was the unsuccessful one. Telling of each persons path may be instructive to understand how the system didn't really work back then unless you came through in a specific way.
We were all within a couple seconds PR at all distances when I was a JR in HS (one guy was a SR). He ended up running terrible that year, not improving much, while my other teammate and I did improve well as JRs, but far below expectations. The older guy graduated, quit running, and went into the military.
My teammate and I made it to higher levels as SRs. I was lower mileage because I was deemed a 800/1600 guy (based on my great kick even though I was really good at long distance when I had enough endurance and I was only a 202 800 guy). My mileage was cut to 25-30 which killed me at 3200m, but I was state class in 1600m. My teammate boosted his mileage to 40-50 to focus on 3200m and became 15th? in the US with 9:10. I still beat him in the mile and owned him in the 800m but he'd gapped me in the 3200m and cross.
I went direct to Pac 10. As a walk-on I was still within a couple seconds of all the incoming freshman in the 1600m and was recruited so I didn't feel like some joker who showed up with unrealistic expectations. I was planning to move to 5000m since I didn't think I'd be competitive at 1500m in the long run. I knew I had work to do but was excited to get started because my teammate had showed me that it wasn't unreal to reach the higher levels.
My same age teammate went to JUCO and ran for a guy who is now one of the best coaches in the country. He spent a year there and transferred to my Pac 10 school with a 14:30s PR at 5000. He had the advantage of already having some endurance development and running with a pretty good team of guys and for a coach who developed him to the next level instead of putting him through a meat grinder.
The older guy (in military) started training for a marathon doing lots of easy mileage and some threshold type work. Started PRing a lot at road distances.
My freshman year in D1, all I did was a lot of sub 6:00 pace runs, which I could do in HS (but rarely did). I could keep up with the All Americans on road runs, and was encouraged to by our coach. Sometimes we'd dip into the 5:30s in the middle of a 11 mile run. On interval days I was mystified at how bad I was running thinking "I'm doing a lot more distance (40-50) and way harder so why am I sucking here?". I absolutely never had a single good interval workout in college. I didn't PR in anything that year. In interval workouts when I tried to pace myself (look at watch), coach would yell at me and tell me I should be looking at the group and stay attached.
My same age teammate transferred in sophomore year and made the top 7 in XC, as he was easily running 25:00 8ks which was good enough then. I was running 27 high 28 low and maybe 12-13th best guy; making the top 7 you'd need 26:30 (we were top 10 in the nation twice and top 20 the whole time I was there so this team was very good). My teammate was able to run at what I'd call the varsity level through his whole college career but stagnated pretty much the whole time he was in college and was never All American or an NCAA track qualifier. He'd later go on to run 4:06y, 8:11, 13:59, and 29:59 in the first year or two out of college, then that was it for him. He was the slowest of the three of us in the mile in high school.
I never improved and began experimenting with higher mileage, not knowing that I should slow down my runs, so I just brutalized myself into oblivion. I transferred to another Pac 10 school after 3.5 years because I was so miserable in both school and running I just needed to start my life over. I quit running for a semester, then started training for a marathon and on a run in the summer I ran into one of my new school's guys who said "we need you on our team". They really didn't but it was nice so I decided I might as well give it a try. At this school everyone was pretty much self-coached (coach non-present) and I ended up rooming with an African superstar so I could learn all I could about what they did. I learned that they did a lot more miles so I started doing that but still at 6:00 pace which I could survive but it was not developing into a better runner. I ended up running low 27s in cross but being 8th man so I didn't make the top 7 (this was a team I totally should have been able to make if I had any idea what I was doing). In indoor track I PRed pretty well in the 3000m but this was my last year so it was all over anyway. I planned to graduate and move on to marathoning.
During all this time, the military guy just kept improving and improving running marathon training and steady tempo runs. He got out of the military, went JUCO for 2 years, then went to D1. I believe he was All American a couple times, won conference championships, and eventually went pro (say B level) in the marathon. He eventually made a World Championship team and qualified for the marathon trials a few times.
I graduated, started working, lived in Park City, and started training longer. Moved down to SLC after a few months, had a great year of training (could run 10 miles at 5:30 pace on the track in SLC for tempos) but still wasn't seeing better race times (I never ran over 6:00 pace so I literally never rested). I eventually started working overseas and there was no running scene there so I quit thinking my goal to make the marathon trials was unrealistic (2:22 standard then).
About 6 years later I discovered letsrun and read about training here and realized I'd never trained properly in my life. It was too late for me but I believe the internet has revived distance running in America.
Sorry for the long post but it's hard to summarize how some people could progress in the 90s while others would hit a brick wall.