Start in a traditional plank with your shoulders directly over your hands and wrists. Be sure to keep your back flat and your butt down, maintaining a neutral spine.
Engage your core (think about pulling your belly button toward your spine) and lift up your right knee, bringing it forward toward your elbow. It's OK if you can't bring your knee all the way in. Return your right knee back to the starting position as you simultaneously drive your left knee up toward your left elbow. Return to the starting position.
Continue switching legs and begin to pick up the pace until it feels like you're "running" in place in a plank position.