Stand with your feet hip-distance apart. Hold two dumbbells overhead (or a plate or medicine ball if you have it), locking out your arms and engaging your core.
Keeping your belly tight, take a step forward with your right foot, coming into a lunge with both knees at 90-degree angles. Gently tap your left knee to the floor.
Immediately step your left foot forward, coming into a lunge, gently touching your right knee to the floor.
This counts as one rep.
Keep walking for 40 seconds, alternating legs. If you don't have room to walk, do alternating lunges in place. Rest for 20 seconds.