Find a stable, waist-high surface that can support your weight, such as a bench, table, couch, bed, counter, wide windowsill, or chair. (You can also stabilize a surface like a chair or table by pushing it against the wall.)
Plant your palms on the surface, making sure your wrists are right below your shoulders. Step your feet back until you're balanced on the balls of your feet in an elevated plank position, with your core pulled in tight and your body in one straight line. Hug your elbows inward to activate your back muscles and lift your kneecaps up toward your hips to engage your quads.
Drive your right knee up toward your chest, keeping your upper body stationary and core pulled in.
Lower your right foot back to the ground as your simultaneously drive your left knee up toward your chest. You can either speed this up to a quick, running pace, or continue more slowly, placing your foot fully on the ground before driving the other leg up. Choose a pace that feels good on your joints but is challenging for your muscles and gets your heart rate up.