I think you have to look at your diet. If you are unsure about your salt intake, go to a sport dietician. You won't be sorry-I know from experience. I participated in numerous triathlons and had cramps all over the place.

Are you taking supplements like creatine? If so, you may have to increase your water intake or stop it all together.

Are you stretching properly and enough?

Are you calves strong? If not, you need to exercise them in gym.

How are you building up to your runs? Are you timing your meals and fluids properly?

Hope those pesty cramps stop and that you get to that marathon level without further problems.

Happy training