As a runner it is important that you keep your body fueled with the right nutrients and vitamins. Runner's burn a high number of calories in their training and therefore need to take in more carbohydrates and proteins than non-runners to be able perform well.
Carbohydrates are the best source for endurance runners as it help the body to store, preserve and replace muscle glycogen (the body's of energy source). A low level of glycogen will result in fatigues or low levels of energy on race day. Once all energy sources have been depleted, the runner is of risk to 'hit the wall'.
Therefore, before you tackle the all important race day, ensure you increase the overall carbohydrates intake 2-3 days before race day and reduce fat and protein.