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When training for a long run 13.1, 26.2 or more miles, it is about covering the distance more than simply running fast. The major determinant on whether or not distance can be covered depends on glycogen stores. Glycogen is stored glucose and your muscles' preferred fuel for aerobic exercise. When endurance athletes "hit the wall," they have depleted their glycogen stores and have to rely increasingly on fat as a fuel source. Unfortunately, when you have to burn more fat, you slow down. In some cases, Stop! And even when you don't stop, you may run abnormally, causing increased muscle soreness or injury.
Most people can store only enough glycogen in their muscles for 15 to 20 miles. However with training, you can extend the distance that your glycogen stores will last. Because your body can adapt specifically to the stress you put on it and during endurance training you are actually simulating the demands of the race. For half/full marathon training this means distance, distance, distance. Nearly every finisher on event day will have completed at least one 10 or 20-mile run in the weeks before tapering. What these long distance trainings achieve is ability to condition your body to use more fat relative to carbohydrate at a given pace, which allows you to run farther before you run out of glycogen. You become more “fuel efficient”. The training also teaches your muscles to store more glycogen. Because, when you do a longer distance, you deplete your glycogen stores. Your muscles are then stimulated to restock your glycogen at a higher level. This is a simple survival mechanism to ensure that you won't run out of glycogen again. In other words, your gas tank becomes bigger. Finally, your fuel-delivery system improves, moving nutrients in and waste products out more efficiently.
Because these adaptations take time to occur, training for a marathon isn't like cramming for a test. Most runners do their last long run two or three weeks before the event, then dramatically reduce their training. During this time, their muscles retain the ability to store a lot of glycogen but aren't being drained. In the few days prior to the race, a high-carbohydrate diet or "carb-loading is used to top off the tank.
NOTE: One of the biggest obstacles to marathon success for properly trained runners or walkers is improper pacing. Once you combine race-day nerves and rested legs from tapered training, many runners have a hard time not running the first several miles faster than they should and deplete their glycogen faster.
Nutrition and Glycogen replenishment
Think of your body as a gas tank that carries and stores fuel for you to “run” on. For example, if you were getting ready to drive a long distance, you would top off your tank so you wouldn't run out of gas. Well your body is the same way. On run days, especially long distances and increase days you want to ensure that you have topped off your “glycogen fuel tank” with carbohydrate-rich foods to ensure adequate storage. As distances get even longer sometimes like a road trip you have to stop and get gas half way to your destination. Again your body may need replenishment before the end of your run and that is when carbohydrate-rich supplements are going to be key. Back to our example: so once you reach your destination, you would never run your car on an empty gas tank or your car would not only stop may be damaged. Well again our bodies are similar. You need to refill your “glycogen fuel tank” all the way back to full once your walk/run is complete. This can be done by first immediately following your session with a snack consisting of 200-300 calories of 75% carbohydrate and 25% protein. The protein will help in the healing of your muscles and the carbohydrates to restore some of the glycogen stores. Once you leave your training, within 1-2 hours you want to follow up with a carbohydrate/protein rich meal. Calories for your meal are dependent more on your body size and calories burned. No one can say that everyone should go fill their tanks on 700 calorie meals because that would not be accurate. For more individualized plans, work with a registered dietitian to assist you in your daily caloric needs and those specific needs for training. There are many websites that one can use but not all are endorsed for accuracy so be careful to pick a website that has reliable staff such as registered dietitians.
KEEP YOUR GLYCOGEN TANK FULL!!!
BE CONSISTENT, PACE YOURSELF, EAT WELL