If you want to reach your endurance potential, you will have to train your aerobic system optimally. This means you will be using your slow twitch muscle fibres and burning mostly fats as fuel. Beyond a certain work rate, an athlete will start to burn glycogen stores more than fats, and they will be using Type IIA fast twitch muscle fibres that are not designed for prolonged endurance, and are designed to burn carbohydrates rather than fats. An athlete who hasn’t trained aerobically will run out of high-energy fuels within 2 hours or less as the glycogen that is usually stored with the IIA muscle fibres or in the liver gets used up.
You have to make sure you keep your heart rate very steady, and never too high, to get the best aerobic training effect. At first, this will mean jogging at speeds that can seem far too slow to qualify as true running. Making sure that your effort remains as low as your patience allows, every day, for several weeks.
If you stick rigidly to this deliberate low-intensity daily running routine, after only several weeks you will find that for the same low effort, you will be running faster and more economically. This progress can often continue for months on end, so that eventually, you will be running comfortably and aerobically, using mostly fats for fuel, at speeds that were formerly uncomfortably fast or bordering on anaerobic.