At the start of a new game, go to the "safe zone" of the Ranger Station (the circle on the map where zombies cannot spawn/respawn). When you are in the parking lot of the Ranger Station, either run to the left or right while turning the camera in the direction you are running. When stamina is depleted, stop and rest until it completely refills. Repeat this process to continue to increase your cardio level. This process takes a while, but since the opening section of the game does not contain any tough zombies (plus day cycles do not occur until after you reach the church), this is a great time to build up the cardio stat. Having a high cardio level will give you more stamina for things such as running, climbing walls, and melee fighting.