Today is a day for cardio and since it is summer I strongly recommend swimming. Swimming gives a great cardio workout with an incredible amount of low impact resistance, meaning that it is great on your joints as well. Plus, you get to be at a pool. bonus!For all of those of you who still refuse to do anything outside of skating to train for playing roller derby, maybe try skating underwater. This is no more ludicrous a notion than the one that makes you believe that all you need to do to be able to excel a sport is skate a lot. Best of luck!