Cardio is an aerobic excercise and usually aerobics tend to take away from explosiveness which in the game of football is key. Ive been Jumproping for 5-10 minutes everymorning and sometimes more at night. Once or twice a week i do a cardio ab workout that takes about 30 minutes (10 minutes walking fast, then abs, 10 minutes jogging, then abs, then 1 minute intervals of jumprope and walking then abs). Let me know if im over doing it on the cardio.