Extreme endurance sporting events, such as the Marathon running race, place large demands on muscle glycogen stores. During exercise, the muscle and liver glycogen stores will be fairly depleted after about 90 minutes of exercise, even sooner if the diet has not been adequate in the lead up to an event. Ideally in preparation, an athlete should aim for a daily intake of 8-10 g of carbohydrate per kg body mass, along with minimal amount of exercise in the 2-3 days prior to an event, to increase the glycogen storage capacity of your muscles, and hence improve your endurance performance.
During these days you should also be aiming to reduce fiber intake and increase fluid intake. You may see an increase in body mass by about 1-2 kg over this time, because with the extra storage of carbohydrate there is also storage of water in the muscle, which increases your weight while helping to keep you hydrated.