We need to look at the athlete before we look at the training. With Coe, I think talent and childhood preparation overruled the need for high mileage.
In Winning Running by Peter Coe, a lot of emphasis is given to cross country seasons while middle distance athletes are young. This may be where Coe built up his base for his adult years, though I don't know the specifics of Coe's mid teenage racing/training.
I also thought I read somewhere that his VO2max was in the low 80's, but I can't find it at the moment. (Thought it was Lore of Running). This would indicate the talent to then focus on quality work instead of quantity.
Also note that warmups and cooldowns were not counted as miles by the Coes, which may have made his training more typical of other elite MD runners when that mileage is considered.