Help with fitness regime for developing stamina?

I'm 6' tall but really thin and underweight. My problem is that whenever I play football( and I do so a lot), I can only manage around 20 minutes of good sprinting before I cramp up around the calves. This is really frustrating and I often end the match hobbling off the pitch. What kind of exercise schedule and diet should I follow to eliminate this? Would also appreciate tips for gaining weight (I eat a lot but my metabolism is high and my digestion is weak)