Triathlon training really isn't just tacking together the training you would do for all three sports; this would be impossible. Instead, it is doing the same amount of training as you would do if you were doing one of the three sports exclusively. But in the same amount of training, you are training to perform three sports rather than one.

This necessarily means that a triathlete is not performing to his/her ability in any single sport. For example, a good time for the marathon portion of an Ironman is 2:40-2:50, but a competitive marathon time is 2:05 or faster. Just to add additional perspective, a good marathon time for an amateur runner is 2:30, faster than what a topnotch triathlete would be expected to pull off.