When starting a new game, head to the Ranger Station until you're in the "Safe Zone" (aka that circle on the map where zombies cannot spawn/respawn). Once in the parking lot of the Ranger Station, either run to the left or to the right while turning the camera in the direction you're running. Once stamina is depleted, stop and rest till it fills back up. Repeat and watch the Cardio levels rise! This will take some time but seeing as the opening section of the game doesn't have any zombies worse than the normal ones (plus day cycles don't occur until AFTER you get to the church) this is a great time to build up this particular stat, thus giving you a heads up seeing as how important stamina is for such things as running, climbing walls, and melee fighting.