80/20 running is a term popularized first by Matt Fitzgerald, although this general concept can be found in many different training program types. Briefly, this theory advocates that 80 percent of your running should be done at an easy or Zone 2 effort and 80 percent should be done at a higher intensity.

Running the majority of your miles at easy effort is the key to building aerobic capacity and endurance over time. The benefits of easy running include allowing your body  

As you start to increase your mileage, the ability to adequately recover is one of the things that keeps you healthy and injury free. If you are running all of your mileage at high intensity, the overall stress and lack of recovery significantly increases the chance of injury.

Easy running also promotes aerobic and muscular skeletal physiological development. On a cellular level, it fosters growth of additional mitochondrial, frequently called the ‘powerhouse’ of the body. It trains your heart and circulatory system to move blood more effectively through the body.

Running is a high impact sport. Equally important to your aerobic development, is the capacity of your muscles, tendons, and skeleton to sustain the volume of impact from running, particularly as you build volume.  These changes take time. Faster, high intensity running stresses these systems more, increasing the likelihood of injury.

Even for seasoned and experienced runners, easy running has an essential place in their training plan. Running the bulk of your miles at an easy effort supports higher training volume and reduction in injury potential.