What happens with my muscles when running long distances regularly?

Running is an exercise that is of relatively low intensity but of long duration. This kind of exercise is called aerobic exercise because your muscles can depend on oxygen acquired by the lungs to produce enough energy (ATP) in the muscles required to do the work.

Regular running produces an increased number of mitochondria in the fast-oxidative and slow-oxidative fibers which are activated in this type of exercise. There is also an increase in the number of capillaries around such muscle fibers.  All these changes contribute to an increase in the capacity of your muscles for endurance exercise with fatigue setting in much later.

Increasing your muscle’s capacity for endurance exercise causes muscle fiber diameter to decrease slightly resulting in a small decrease in the strength of these muscles. You can see this by comparing the smaller physique of a long-distance runner to a sprint athlete who has a bigger physique with more bulging muscles. Sprint training is anaerobic in nature and muscles react differently to this kind of exercise.

Endurance exercise produces changes in not only skeletal muscles but also in the respiratory and circulatory systems. These changes improve the delivery of oxygen and fuel molecules to the muscle.