Along with balanced diet and exercise and proper training and practice, keeping a control on your mind is also equally important. You need to develop a strong mind and will power, an attitude to not give up at any point of time, little bit of time management, deep breathing techniques which will help you when you run the marathon. Keeping a control on your breadths is an important part of running exercise. Practice yoga and meditation to keep yourself stress free because you need to improve on your focus and concentration while you run and nothing should distract you. All the best!!