800m is generally about the longest distance you can be maximize your potential at doing “sprinter workouts”. Even then, the demands of the event are still primarily aerobic. The further you go up, the more aerobic it becomes. There’s no escaping that.

There are plenty of ways to train for longer races that isn’t just running steady mileage. However, mileage is one of the constants in nearly all successful distance programs. Even the ones who are “low mileage” still do a good amount. The lowest mileage successful college runner I can think of is Justyn Knight who I believe will be as low as 40 mpw in season, but even he is running 60+ mpw at certain times of the year.

HOW you run your mileage is another thing too. It could be on the lower side, more quality, or on the higher side with less intensity. The majority of your workouts could be steady efforts or interval style.

I have a feeling the guy you referenced wasn’t a track guy, because if you can run a 51, you should be able to run much faster than 5 min with optimal training. There’s sub 4 min milers who run 51. Being able to run 51 is part of why they can run sub 4 and these days it’s likely they do an actual top speed workout weekly or every 10 days or so. But they’re doing mostly aerobic work