(Simple)
For endurance athletes like long distance runners, carb loading is essential in particular the week of the race. Thus, runners need to consume a massive amount of starchy foods to sustain the energy levels necessary to reach the finish line of the race.
The function of glycogen
Carbohydrates are well known as a main source of energy. However, glycogen, the stored form of carbohydrate in your muscle, is also a source of energy for your muscles to move. You should increase carbohydrate intake prior to the race as your body burns glycogen when you run hard. And the good thing when loading up on carbs before the race is you can delay fatigue longer as long as your glycogen stores are at full capacity.
What carbs should you eat?
Easily digestible carbs such as white potatoes, cereals, pasta are the best pick to top off your glycogen levels. Yet, avoid eating higher-fiber fruits and vegetables since higher-fiber foods are slow to digest.