I think there are a lot of people who would do equally well , or nearly so, at the 800 on a high, or low mileage program, more than any other event anyway.
The 800 comes down to what are your strengths and weakness, and then figuring out which are the best to attack the most with training. One of Juantorena's obvious strengths was just that , massive strength. I don't think he dabbled in the gym . A lot of mileage would have just warn him down. Obviously, he was gifted with the endurance needs of the event too. A big guy has to be because , like I said , the mileage route to obtaining aerobic endurance would wear him down. Even if he could have been equal at the 800 on a mileage program, which is possible, he wouldn't have been the 400 gold medalist taking that route.
Of course he is an extreme example , so most people shouldn't assume they can do the 800 best by training by his example of low mileage and lifting and plyo like stuff, that I assume were pretty severe. He proves that it can be done by the right animal. Some people are at other extremes and benefit from mileage, Heavy weight lifting, too much intensity in running workouts, would lock them up, lead to running mileage dropping off a cliff and not allow them to hold on to and build their own strengths. Other athletes are somewhere in between.