Here is an applicable quote from a letter to U.S. coaches from Arthur Lydiard:
"Maintaining a lower-than-normal blood pH for long periods of time, as a result of excessive anaerobic training and build-up of lactic acid in the muscles as the net result, can destroy the potential development of the runner, especially the teenage runner. Whereas the Africans would be running leisurely, trotting to and from school, their counterparts in the U.S.A. would mostly be on the track doing hard anaerobic repetitions. This is where the real problem lies."