In my experience, both with my own personal training and that of my athletes, most athletes are more likely to do a given workout on their plan if they know the specific purpose behind it, or at least that there is one, and that it will benefit them. Not only are they more likely to do the workout, but they are more likely to do the workout as it’s written.  For some endurance athletes the former is periodically an issue but much more frequently the issue is with the latter.  A good training plan isn’t random in where various workouts are placed or with what specific intensities are called for at different times.

Aerobic exercise does many things to the athlete’s body.  It increases the maximum amount of oxygen that can be used in a given amount of time through multiple means.  A few examples of how this occurs are increased cardiac output, greater capillary density in the muscles being trained, and the increased size and number of mitochondria.  Other physiological changes occur as well, such as improvements in the body’s cooling systems.  Anaerobic activity, on the other hand, does not improve those things.  It does improve other aspects of performance, though.  Some of those, such as improvement of the creatine-phosphate energy system, have basically no beneficial effect for endurance athletes.  Others however, such as improved neuro-motor connections, do. 

Just by understanding the basics of what I’ve explained above, I have seen some athletes get noticeably better at sticking to the prescribed durations and intensities of their workouts.  Add on top of that the different zones within aerobic training that stress certain physiological pathways to different degrees.  To many endurance athletes, myself included back when I was getting started with triathlon, there is the idea that every workout needs to be hard and leave the athlete gassed.  One big problem with this approach is that the easy workouts aren’t done easy enough, and then because of that lack of recovery, the hard workouts aren’t done hard enough.    What ends up happening in this all too common situation is that many physiological systems aren’t trained at all, and the ones that are aren’t given the recovery needed to make adaptations as well.  The end result is often a hard working athlete who’s not seeing the performance gains that they might have the potential to make if they trained differently.