Rigorous endurance training.
A marathon is an endurance race that needs a lot of mental and physical preparation. The old adage is true that says: "The race is run between the ears." It simply means that a lot of mental preparation is needed. This is important as the rest of the body is governed by a sound mind. In addition, more physical training is need to make sure that the muscles can stand the strenuous action. This prepares the oxygen reserves in the muscles.