Running 13.1 miles in a half marathon race is a long way, and along the way you will burn a lot of calories. Depending on your size and effort, generally you will burn approximately 100 calories per mile when you run. So you should burn around 1,300 calories during a 13.1 mile run, and because of this, it is important to understand that you need to consume enough calories to run without "hitting the wall" or running out of energy.
Calorie Expenditure
Jim Fixx notes in "The Complete Book of Running" that you can expect to burn anywhere from 800 to 1,000 calories per hour of running. This amount can change due to certain variables as you run a half marathon. He points out it's not necessarily your pace during the race that determines how many calories you burn but your weight. He gives the example that a 220-lb. person running an eight-minute per mile pace burns 150 calories per mile, while a 120-lb. person running at the same pace uses only 82 calories per mile.
Calorie Storage
Calories, especially carbohydrate calories, are the stuff that fuels your muscles as you run, so proper caloric storage is essential. In Pete Pftzinger's Lab Reports, it is noted you can store 1,600 to 2,700 calories in your muscles as you get ready to run your half marathon. This is one of the things that distance runners enjoy, eating as much as they want to fuel their muscles for an upcoming race. Theoretically, if you are burning approximately 1,300 calories during the half marathon, proper eating or carbohydrate-loading should provide you with enough calories for the entire race.
Carbohydrate Loading
Pfitzinger points out the importance of storing enough calories for your long-distance race via carbo-loading. The three days before your half marathon are important days for you to eat enough glycogen or complex carbohydrate-loaded foods. This gives your muscles enough time to convert and store those calories into burnable fuel. Without this type of eating to fuel your body during your run, you will run out of energy before you see the finish line. He also suggests fueling during the race by drinking carbohydrate-loaded sports drinks, and using energy gels to top of your fuel tank.
Caloric Usage
One important aspect of the calories you burn as you run is their source within your body--your main fuel of glycogen calories stored in your muscles, or calories of fat that are your reserve fuel. Tim Noakes in "Lore of Running" points out that the harder you run, especially early in the half marathon, your body will use up almost all of your complex carb calories rather quickly. Once these calories are used up, your body turns to fat calories to burn for fuel, and these calories are not burned as quickly and are less efficient, causing a decrease in your performance.
Replenishing
With all of this data on the amount and type of calories you will burn while running a half marathon, it is important to factor in the difficulty of your training. If you have been burning more calories than usual because of some difficult workouts in preparing for the race, you may be in a calorie deficit. With this in mind, you need to consume more calories than is typically recommended. Monitor how your body is feeling prior to the race. If you are feeling lackadaisical and listless, it's time to chow down and consume more calories.
References
- Pfitzinger Lab Reports: Eat, Drink and Finish Strong
- "Lore of Running"; Dr. Tim Noakes; 1991
- "The Complete Book of Running"; Jim Fixx; 1977
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