Ask any runner or coach what the most critical component of a marathon training plan is, and the unanimous answer will be the long run. That’s because the long run’s physiological benefits are indisputable.
First of all, it strengthens the heart: Ventilatory capacity—the ability to move oxygen in and out of our lungs—increases. The musculoskeletal system adapts and grows stronger thanks to long runs. Endurance improves because mitochondria (the energy-producing structures in cells) and capillaries (tiny blood vessels that transfer oxygen and waste products into and out of cells) become more dense the farther you go. You also get better at using fat, rather than glycogen, as a fuel source. The list goes on.