Fatigue during the race is, of course, inevitable but it is possible to delay the onset using certain techniques and training adaptations. For endurance sports such as marathon running, the body will use its stores of glycogen to fuel it for energy, which will run out after approximately 90 minutes. Therefore, it is essential to keep glucose stores up throughout the race. It is recommended to have 30g-60g of glucose per hour to maintain adequate blood glucose levels. You can use gels or small high carbohydrate snacks if possible. Eating during a long run should be practiced in training on not on the day of the marathon.
Eating adequate carbohydrates (5g+/kg bodyweight) and tapering training in the days prior to the race will enhance glycogen stores and metabolic adaptations throughout your training season will help direct your body to burn the abundant stores of fat as fuel, thus sparing valued muscle glycogen in the race. Protein will help muscle repair and immune function so is a hugely important part of recovery meals.
Hydration and electrolyte replacement will also play a role in offsetting fatigue. Replacing lost electrolytes with a sports drink with carbohydrate will ensure you are refueling lost water, salts and glucose.
It is critical during your training and especially in the days before the marathon that you are resting adequately. In training, always have a rest day and replenish your body with nutrients such as fruits, vegetables, carbohydrates and protein to aid repair and recovery.