Genuine High intensity training is simply too hard and too painful for most recreational endurance athletes and most serious ones are happy to commit lots of time to their physical pursuits. It is not a surprise, then, that almost all endurance athletes seem to favor traditional training programs that focus on volume at lower to moderate intensities. Higher intensities are typically only used by elite athletes with superior recovery abilities and even then, used sparingly. There is good reason for this - adding high intensity training into a program with typical endurance program volumes creates an extremely demanding program only suitable such athletes. Sure, some non-elites will occasionally use high intensity effort, and many will emulate the programs of the elites by using higher intensity work (not really the same thing). But my experience has shown me that--assuming i'm somewhat typical of the ambitious amateur athlete--adding genuine high intensity work into an endurance geared training program using traditional volumes is, well, problematic. There are just too many excuses.
However, at least in my opinion--it is clear that we can only reach our actual physical potential in terms of endurance training by including high intensity work. High intensity training is what gets you faster. Increases in speed always have corresponding increases in endurance but it is a one way street--increasing endurance will not always increase speed. High intensity work also develops a capacity for intense but limited duration suffering that is critical for strong and fast finishes. If you want to race long and you want do it as well as you can, you will need to develop your ability to consistently perform high intensity workouts.
In my experience this is far from easy--even under the best of circumstances. Those interested in learning how to perform high intensity work and developing their ability to do so consistently might be best served by spending time on a program such as the one i'm doing now (3 x 8 minute sessions a week, one each of biking, rowing, and running). For an accomplished and eager athlete, elimination of all efforts other than high intensity work might provide the needed incentive to really learn what high intensity work feels like. For someone used to 5 or 6 hours a week, anything less than a maximum effort during these three weekly workouts would feel like 'selling out'. The athlete would, after even one sub-par performance, feel like they weren't really working out at all - caged and restless. Such restlessness, combined with the ample time for physical recovery based on such a low volume program, helps create an environment in which many of the mental and physical obstacles to experiencing genuine high intensity work are absent. This is the ideal environment in which to begin to cultivate the relationship with mental and physical anguish that defines these efforts.
If you are able to learn how to feel completely empty after an 8 minute workout--drained mentally and physically to near the point of actual physical incapacity (for example i sometimes finish my bike workouts in such a state that i am actually unable to walk down stairs without falling on my face for up to 10 minutes), then you have essentially developed a new fitness tool. This tool can then effectively be used in conjunction with more traditional methods and intensities in pursuit of your true fitness potential, if and when you decide to make this your goal.