In a one-hour event, 98 percent of your endurance energy is derived from the aerobic system; in a two-hour event, 99 percent is. Does it make sense to spend so many hours a week on anaerobic work when 99 percent of your race energy comes from the aerobic system? Instead, it’s best to first derive your endurance speed from aerobic training, then, as time and energy permits, to add anaerobic training.