Although minutely small, mitochondria are of major importance to athletic activities because the increase in their density is associated with enhanced performance capacity ("Mitochondria Functions" n.d. para. 1).
To answer the question on which between a world-class runner and a typical couch potato would have a much higher density of mitochondria from their sample muscle biopsies, it would definitely be the world-class runner because mitochondria is concomitant with exercise and physical activity.
Furthermore, mitochondria are the only places inside the muscles where carbohydrate, fat and protein can be broken down in the presence of oxygen to create energy needed for exercising. As a consequence, the more mitochondria an individual has, the more energy that individual can generate during exercise resulting in faster and longer time doing running, swimming and other physical activities ("Mitochondria Functions" n.d. para. 2).
Several studies have been done to document or otherwise prove that increased exercise generates higher density of mitochondria. There are differences from these findings as to what type of physical activity generates more mitochondria such as intensity, faster but shorter duration or longer but slower duration.
There are three types of human muscles: the cardiac, smooth and skeletal.