Sometimes I would run four 400s averaging 49-50 seconds with about 20 seconds between each one. Sunday was a long run. My training schedule consisted of one hard day followed by an easy day. My easy day was often speedwork.
I would think 2 X 49-50 with a 20sec rest would be challenging enough, let alone 3 with 20sec rest. Four X 50 with 20sec...no, can't believe that. He should have been a 1:40 guy if he could do that workout.
Also at 50miles a week he likely was counting his mileage days but not warmups or warmdowns and not his reps. 50miles a week is nothing for a world class runner.