Day 15 is a list card of pure running: 10 march steps alternating with 10 high knees, five times through, for 50 of each per set, then 2 minutes rest between sets. No floor, no jacks, nothing but knees coming up at two different speeds.
The march and the high knee are the same movement at two tempos, and alternating them ten reps at a time is interval training in its simplest form. The march steps are the recovery interval - controlled, upright, knees to hip height - and the high knees are the push, the same knee height with a fast bounce. Because the recovery is still work rather than standing still, the pulse never comes all the way down, so a set of 100 reps ends up feeling longer than the number suggests. It also makes this one of the program's recovery-on-the-go days: heavy on breathing, light on impact.
Make the march steps genuinely slower than the high knees - if the two blend into one pace, the intervals disappear. Keep the knee lift the same in both, hip height every rep. Stay on the balls of the feet for the high knees and pump the arms; keep the upper body tall in both, no leaning back to help the knee up.
Fifty slow, fifty fast, then rest: the whole session is a lesson in changing gear.
Make it Harder: Lift the knees above hip height on every high knee, aiming for the chest.
Make it Easier: Do the high knees at march pace with a bit more bounce and keep the count at 10 per block.
