In a non technical way the muscles have turned into fat and you need to burn that away. Normally, in endurance events, you supply the energy with carbs but at some point, if you push yourself, you will need to burn fat and convert that into energy. Fat is not an effecient energy source, and you need to train to burn fat, as it not pleasant and in an event quite painful. Any endurance sports or higk rep in a gym should do that - just watch your food consumption also as the fat likes to replace itself. Good luck and happy training.