How it works
The human aerobic system is comprised of: heart, lungs, arteries, capillaries delivering blood to muscles, blood, mitochondria (little factories inside muscle cells that utilize oxygen to create energy from metabolic fuels.) Running creates changes all of these parts of the body to improve aerobic capacity. They key ingredient for strengthening the aerobic system is stress.
High-intensity exercises strengthens the aerobic system faster than low-intensity exercises. A study conducted at McMaster University examined the effects of exercise on mitochondrial protein synthesis in muscle cells. These proteins are used to produce new mitochondria in order for the muscles to use oxygen faster to fuel their work. The faster these proteins are created, the more dense and packed with mitochondria the muscles become. A single high-intensity work can increase mitochondrial protein synthesis much more than low-intensity workouts. High-intensity workouts also increase contraction force of the heart more so than low-intensity workouts can.
Low-intensity exercises have their benefits as well. Studies have shown that during prolonged running at low-intensity, muscles release large amounts of interleukin 6 (IL-6), a neurotransmitter that contributes to fatigue. Well-trained runners produce less IL-6, one reason why they are more fatigue resistant. Long slow runs produce higher levels of glycogen depletion and therefore IL-6 release, than short, fast runs. The farther a person runs, the more fitness-boosting IL-6 his/her muscles release.
High-intensity running is different. Large amounts of high-intensity runs are stressful to the body, to the point that it suppresses the parasympathetic nervous system. The consequences are chronic fatigue and loss of performance. But it's not about what type of training is better. It's about how to adjust the ration of each training, which is what the 80/20 training method can help you to do.
We know that inducing fatigue in the body with exercise triggers adaptations, making the body more fatigue resistant. Brain fatigue works similarly. Long low-intensity runs strengthen the cortices in the brain involved with fatigue resistance. The insular and temporal lobes perceive the physical elements of emotional states (discomfort and extreme effort). The Anterior Cingulate Cortex resolves internal conflicts (desire to keep going and desire to quit when running in a fatigued state). The stronger these areas of the brain become, the slower a runner's sense of effort will increase in training and in races, and the longer they will be able to sustain and endure severe fatigue. It's not about how hard the muscles are working, but how long you put the brain to work to stay on a task at hand.