The program opens the way it means to continue: three exercises, twenty seconds each, performed back to back for one solid minute of work per set. High knees, squats and butt kicks make up the circuit, you rest for a minute after each round, and the level you pick - 3, 5 or 7 sets - decides how big your first day is.
The design is a running sandwich. High knees spike your heart rate first, squats shift the load to strength while your pulse stays high, then butt kicks return you to running form with the emphasis flipped from knee drive to heel speed. Nothing here needs learning, which is exactly the point - Day 1 is about finding your flat out setting, not mastering new movement.
Drive the knees to hip height and stay light on the balls of your feet in the first interval. Squat with thighs approaching parallel, weight in your heels, arms reaching forward for balance. On butt kicks, snap the heels up toward your glutes with a quick, compact stride. The transitions carry no rest, so flow from one exercise straight into the next.
Day 1 doesn't ask how fit you are - it asks how hard you're willing to go for one minute at a time.
Make it Harder: Sprint the final five seconds of every interval - all three exercises, every set.
Make it Easier: March the high knees and butt kicks at a brisk walking pace and keep the squats as written.
