Veg*n Thinking of eating a lot less meat.

Hey guys, lately my side stitches (cramps) have been acting up when I run. Let me tell you that running a 10k for me ( a teenager ) is extremely hard at the moment. I remember when my grandfather started running marathons and would have the same pain. It turned out that in a few years he would get diabetes. It was a choice for him that he could keep eating meat and stop running or he could become a vegetarian. He became a almost a full vegan with the exception of eating dairy and meat every few months. I would like to limit my meat that I eat and slowly start not to eat any at all. Could you please help me with this and recommend me some foods or substitutes to get me on the right track? I would love to pursuit my goal of running at least one marathon in my life and with my stitches in my stomach it will make it harder to achieve. Thanks!