DURING RACE CARBS
Your body can only store around 2000 calories (~8400kJ) of energy from carbohydrate. Once this is depleted, you will no longer be able to sustain the intensity you were performing at when fuelled by carbohydrate. Consume familiar sources of carbohydrate during the race (i.e. practiced in training) A good rule of thumb to go by is consuming a source of carbs (such as a gel or sports drink) every 30-40mins.
Its important to remember there is a big difference between fuelling to optimise performance, and fuelling because you need to. Sure, you might be able to run 21.1km without taking anything on board, but, will you run faster if you do? Most likely you will.