When it comes to strength training, most coaches and even clients understand the concept of progressing in a structured and systematic way, using percentages to build strength slowly over time.
But, for some reason, when it comes to metabolic conditioning, fitness becomes a free-flowing ocean of random, high-intensity efforts, filled with varied, whatever modalities are en vogue that week:
Read The Four Cs of Metabolic Conditioning at its original source Breaking Muscle:
http://breakingmuscle.com/fitness/the-four-cs-of-metabolic-conditioning