As your body becomes more adapted to aerobic, slow runs, it’s going to use fat more efficiently. This process is known as the fat adaptation effect.
Faster anaerobic runs upwards of two hours mainly deplete stored muscle glycogen from carbohydrates. Slower aerobic runs, on the other hand, use approximately 50 percent fat for energy while the remaining 50 percent is a combination of glucose and protein for energy.
Fat oxidation requires oxygen and it’s very hard to run long distances at an all-out fast pace.
Long, slow distance runs are easier to sustain. So during these runs, your body has to constantly replenish the oxygen reserves it is using to continue to produce energy and since fat metabolism requires oxygen, you condition your body to use fat as its main energy source rather than carbs.
Eventually, this adaptation will allow you to run longer distances without having to refuel.
Easy runs also train the cardio, respiratory, and muscular systems to work more efficiently, they allow the body to better integrate its various systems and in turn, this will allow you to run with less effort on your faster running days.
Slower runs also train your slow twitch muscle fibers, the ones that allow you to work aerobically to sustain your pace on long distances. And while faster running is more likely to build up your muscle, slower running is going to help your tendons, ligaments, joints, and bones adapt to the stress of running.
Easy running also strengthens tendons, ligaments, joints, and bones without causing immediate stress on them, lowering your risk of injury. Easy running, despite often not feeling challenging at all, does provide a plethora of benefits for runners ranging from mental too physical and at the end of the day the goal of running is to energize the body and not to break the body down.
With easy running comes the concept of high mileage. High mileage won’t necessarily improve your aerobic or anaerobic threshold, VO2 Max or general fitness, but it will improve your ability to use fat as an energy source and this is key for racing a marathon.
High mileage helps you better utilise glycogen, the starchlike substance stored in the liver and muscles that changes into a simple sugar as the body needs it. Glycogen is the preferred fuel for running, but your levels can become depleted within 60 to 90 minutes. Thereafter, your source of fuel is fat, which is metabolised less efficiently.
High mileage running in essence teaches your body to burn more fat along with glycogen, stretching your reserves from 60 to 90 minutes to 2 hours or more. Top marathoners are probably so efficient in metabolising both fats and glycogen throughout the length of their races because of their vast volume of their training they probably rarely deplete their stores and as a result, they don’t hit the wall.
High mileage training may also result in a more efficient use of your muscle fibres. When a runner doubles his training mileage, we do not necessarily see a change in maximum oxygen uptake, but we do see an increase in the bodies ability to deliver oxygen to the muscles. With improved muscle fibres we see better performances.
"the mileage game"