Start in a lunge position with one foot forward and flat on the ground, supporting yourself with the opposite arm (ex. left foot and right arm forward). Your other leg should be behind you, balanced on the ball of your foot. Kick your front foot back and bring your back foot to the forward position while switching your arms. Repeat as quickly as you can.
Advanced version: Ball your fists and hold your arms in a running position for some added resistance.
Brett's Tip: Imagine you're in a room with a very low ceiling. This keeps your head from bouncing around and forces you to tax your lungs, not your legs.