Glycogen, the sugar stored your body, is a primary fuel for athletes. Glycogen stores are replenished by consuming carbohydrates. Endurance athletes should have high-carb diets. Intricate carb-loading strategies on race week are not worth the risks (e.g., fatigue, injury).
The body has three primary sources of fuel: carbohydrate (carb), fat, and protein. (More precisely, the body uses these substrates to produce energy in the form of ATP.) Fat yields more energy per gram (9 calories per gram (cal/g), versus 4 cal/g for carb and protein ), but carbohydrate is easier to burn. Carb is burned at all intensities of exercise . As the intensity of exercise increases (above about 50% VO2 max), so does the carb-to-fat ratio in energy production .
Animals (including humans) store carb in the form of glycogen, in skeletal muscle (80%), the liver (14%), and the blood (6%) . The human body can store about 400 – 500 grams of glycogen . The body burns carb at a rate of approximately 1 – 2 grams per minute (g/min) at low-intensity exercise and 2 – 3 g/min at higher-intensity exercise . At race intensity, the human body exhausts its glycogen supply after about 2 hours . For marathon runners, this limit is related to the saying that the race is “half over at twenty miles” 
In general, athletes should consume a high-carb diet . During competition, recommendations for carbohydrate consumption range from 30 – 60 grams per hour (g/hr) to 80 – 100 g/hr . Such short-term, high-carb intake requires a mix of absorption rates and glycemic indices . Various race-week diets have been proposed to prime the body to store more glycogen. These diets have been associated with higher rates of fatigue and injury; these risks may outweigh any benefits .
Insufficient glycogen is associated with fatigue and decreased athletic performance [1,3]. It may also elicit chronic overtraining, as follows: At low glycogen levels, the body switches to using more protein for energy production, and it can actually destroy existing muscle to produce protein for this purpose. The resulting muscle damage “interferes with glycogen [synthesis and storage]” , thus decreasing the body’s energy supplies and making future muscle destruction more likely. One athletic doctor cites this as “probably the number one cause of overtraining in athletes” .
- “The importance of carbohydrates and glycogen for athletes“, by Iñigo San Millán (Training Peaks, 2013-01-17). A thorough article on glycogen vis-à-vis endurance athletes.
- “The science of ‘bonking’ and glycogen depletion“, by John Davis (Runners Connect, 2011?). This article briefly discusses the nutritional basics before examining glycogen-depleted training at length.
- “Optimizing glycogen storage“, by Kathleen Deegan (Sacramento Running Association, ????). The takeaway from this article seems to be that stressing about diet the week before an event can be self-defeating. Just eat a generally high-carb diet, and replenish carbs during the event.
- “Carbohydrates, proteins, and fats“, by Adrienne Youdim et al. (Merck Manual, ????).