Lydiard's known for being an early and forceful proponent of base training -- creating an aerobic foundation of fitness upon which athletes can later build. He posited that even sprinters can optimize their performance by training as if they were going to run a marathon, because the endurance training provided a level of conditioning that enhanced the runner's ability to sprint, and gave them a competitive advantage over those that didn't perform base training.
As the author of the article writes:
The first phase of Lydiard training is endurance/aerobic development. Think of aerobic running as home base -- the place where we hang out until we are mature enough to leave, and the place we always come back to for rest and recuperation. As the miles stack up, we increase the capacity of both the heart and the lungs for work, build our circulatory network to the muscles through increased capillarization, increase the number of mitochondria in the muscle cells, and develop other beneficial metabolic and enzymatic pathways for gathering and converting oxygen to energy. Once these structures are established they allow us to respond and recover quickly. Aerobic training, then, is training for all other types of training.To me, this signals the importance of sustained aerobic exercise, whether it be in the form of long distance runs, lengthy bike rides or prolonged cardiovascular fitness routines. For all its benefits, I'm not sure that Insanity -- which is a pure interval routine, with its relatively brief bursts of intense activity -- fits this bill. P90X does a slightly better job of this, in that it features circuit training that builds some measure of endurance. But to really optimize my fitness level, my best bet may be to continue doing some middle- to long-distance running during the week to build up my endurance.