I just wanted to add that an endurance base for the 400m doesn't HAVE to come from slow aerobic work (i.e. distance running). There are other alternatives that might be more effective for a 400m athlete. Developing all-around work capacity through larger volumes of mid-to-low intensity activity is a great alternative to distance running for 400m runners. Aerobic fitness and general work capacity can also be developed through intensive and extensive tempo work.