Do vigorous cardio for 20 - 30 straight minutes using an interval format. For this interval training, do one minute of high-intensity work followed by a two-minute recovery, to stick with a 1:2 ratio. Repeat the intervals until you have completed at least a full 20 minutes; if you don't feel sufficiently challenged, extend your time out to up to 30 minutes. You can either do this cardio outdoors (for example, running or biking), or at the gym (for example, treadmill, spin bike, or stair machine). You can use pace/speed or incline as your method for increasing/decreasing your level of exertion.