We are born with a combination of two types of muscle fibers. Those with a high percentage of fast twitchers can run fast for a short distance, and then become very tired. Fast twitch fibers are designed to burn the stored sugar in your muscles: glycogen. This is the fuel we use during the first 15 minutes of exercise, and it can produce a lot of waste products, such as lactic acid. If  we run even a little too fast at the beginning of a run, the muscles will become very tight and tired very quickly, you will huff and puff, and feel increasingly uncomfortable.
If you have more slow twitch fibers, you won’t be able to run as fast at first, but can keep going for longer distances. Slow twitch fibers burn fat – a fuel that is very efficient and produces little waste product. Long runs will not only condition the slow twitch fibers to work to top capacity as they efficiently burn fat. As you increase the length of the long ones, you’ll train some of your fast twitch fibers to burn fat as fuel. Â
Once the starting pace is controlled (and also the ego), fast runners develop a mix of fast and slow twitchers to do the work of running, and find that they don’t get exhausted at the end. It is the slow pace and walk breaks that keep you in the aerobic zone, allowing you to push back the endurance limit.
Learn More about Marathon Training in Jeff’s Book, “Running – Testing Yourself”