Comment by discreteevent

In fact professional endurance athletes have to train at low intensity because they put out so many watts at higher intensities that they can't get enough fuel in to sustain it for the number of hours they train. If they do then they end up with chronic fatigue etc. Once an amatuer is under a certain number of hours a week (10?) and relatively low wattage it doesn't make sense for them to train at low intensity. (You could be a really fast amateur and still be putting out relatively low watts compared to a pro).