Having done several Half-marathons and haven't had the guts to try a full Marathon i shall part some tips to you. Stretch and do some light jogging/or jogging on spot, 10-15 mins before race. Just jog on the spot and do the some leg stretches and then some abdominal stretches. The abdominal stretches should reduce the risk of getting stomach cramp while running (the pain has nothing to do with the food you eat or the stomach, it's to do with the muscles in that vicinity) but there's no way to actually fully prevent it and could happen to the most seasoned runner or the worst runner (but usually a runner who is not in good form or lacks a good posture). Also search online and you should find foods you should be eating weeks before the marathon in preparation. The morning before eat a light breakfast high in sugar and possibly eat a chocolate bar or two before the race giving you some nice energy for the start of the race until that dwindles away (1 hour or so). Good luck