Aim to do five cardio workouts a week—anywhere from 20 to 45 minutes or more—and make two or three of those vigorous (so you're breathing hard). Not sure how to step up the intensity? After your warm-up, throw one of the following mini-routines into your session (pick any cardio activity).
SPRINTS Crank up the speed and/or resistance and go as hard as you can for 30 seconds to 1 minute. Rest for 30 seconds to 2 minutes, then repeat up to 8 times.
HILL CLIMB Ramp up the incline every minute, maintaining your speed, until you get to a point where you have to slow down. Then gradually back the incline down every minute until you get to where you started. If you want, maintain an easy to moderate intensity for 3 to 5 minutes and repeat the climb.
HILL/SPRINT COMBO Increase the incline and climb for a minute, then hold the incline and go as fast as you can for 30 seconds to 1 minute. Slow down for 2 minutes, then repeat these climb/sprint bursts 4 or 5 times, upping the incline each time.