Cycling: - Do it for 30 seconds at full speed and then cycle normally for 30 seconds.
Leg workout: - This basic, yet challenging, circuit includes squats, single-leg deadlifts, and lunges.
Hill sprints: - Just find a hill, run up, and walk down. Beware, these can be brutal.
Swim: - If you're not used to swimming, doing it for seven minutes non-stop will be plenty.