So you are participating in a marathon race. It is very nice thing and it will no doubt increase your practice and at the same time will prove your physical fitness. First of all start practicing before the race and you should start practicing before more than three weeks. Most people think that they are going to rock the track without practicing and that is biggest mistake that they make. So start practicing. Keep an eye over the calories, once you start practicing you will feel that you are losing weight so you start glutting, don't do that you will increase your weight and won't be able to run.