thank you for nice advice..i will do that tomorrow for my gym day...any way i always have my water near me while im running(in treadmill) but i dont drink because im not feeling thirsty....so maybe i have to drink water even if i dont feel thirsty..
Could be what you feel in your tummy is gas, it can hurt a lot. So, to prevent this is, the normal "rule" for exercise is to wait at least one hour after eating before doing exercise. If you have food in your tummy and you do exercise shortly thereafter, the bouncing around will sometimes create gas, and actually some people will get a muscle cramp in their side.
Could also be another "rule" for exercise isn't being followed, and that would be to drink plenty of water when you're done with your run. If you don't, you'll get dehydrated, and that will make you feel sort of sick in your stomach. Another one has to do with running a long distance without having eaten at all that day. Usually this will cause a collection of acid in the stomach, which can hurt. If you're running right when you get up, then when you complete your run, and you get back to an at-rest stage, have a snack of some kind, to make the acid go away.
I think you will enjoy the marathon, and I hope something I say will actually help your stomach feel better.