Zatopek's workouts were usually something like 40x400 with 200 jog. . . assuming he didn't do any warmup or cooldown, that's 10 miles of intervals and 5 miles of jogging in between intervals.
Where did you get 50 miles a week averaging 6:00 minute pace with Zatopek's training? According to Lore of Running, Zatopek usually did his intervals "around" 5k pace (66-68) with some faster and some slower.
Now I'm not saying he wasn't doing quality, but Zatopek was INFAMOUS for the quantity of what he did do. I forget if it was Bannister or Chataway or someone else, but one of the 1952 British Olympians said of Zatopek, "While we sat around cursing the gods and moaning about how we were too intelligent to train this yard, he goes out in the forest and jogs around for a few hours on his only easy day!"
Sorry, but whoever dominates an event, if you look through the eras, tends to be the guy doing more than his contemporaries. Nurmi didn't do much- but he did a heckuva lot more than his fellow runners. Zatopek might have "only" been running 80-90 a week, but again, much more than his competitors. Snell's 80-90 a week topped the 800/1500 runners at the time, Ryun did even more volume and intensity, Walker did more mileage, then Coe admittedly broke the mold.
And so on, and so on. Just like Kuts topped Zatopek by outworking him, and Schul did the same (no way Schul was low-volume- he was doing 2hr workouts twice a day). Again, and so on.