firstly, calf spasm after running means that you are overworking your muscle strength. it should always be in an ascending manner. start with slow speeds and then increase ur speed.. so also same for the duration of exercise. besides, warm up and toning of muscles is also necessary. most overlooked is stretching! when ur exercise gets over and u don't stretch ur muscles, the lactic acid (waste product) of ur metabolism from the muscles keep accumulating as the tone does not settle down. this causes a trigger for an internal injury and can start a vicious cycle, ultimately causing an "OverUse Injury" another reason is dehydration. take ur pre and post workout weight and drink that much water with electrolytes after ur exercise.