Hi Greevesd

This could be due to your diet. If you have a diet lacking in potassium this may very well contribute/cause the calf cramps you are suffering from. Source's of potassium are found in white meat, root vegetables, fruits (especially banana's) & diary products just to name a few.
Also are you consuming enough water before, during & after your physical activity? You should also try consume a sports drink before & after your running, this will help maintain your electrolyte balance. Several brands make specific drinks for pre & post event hydration. For events of endurance nature it is recommended that athlete's alternate between sports drinks & water.

Hope this helps