Cardio exercise doesn't have to be aerobic. Aerobic exercise is an activity that is done continuously for more than 2 straight minutes. Favorite aerobic exercises are done on the treadmill (walking, running, jogging), bike, elliptical and stair-stepper. Unless you want to run long distance races, you don't need to do aerobic exercise to improve your body's ability to process oxygen and improve your heart and lung function.
Anaerobic exercise is all you need to improve heart/lung function and increase your body's oxygen intake. Anaerobic exercise, like sprinting and weightlifting, requires you to lessen the intensity or rest for less than 2 minutes. Interval training is a great way to do cardio exercise. There is much research to show that interval training sessions are superior to continuous aerobic sessions. You will get better fat loss results and more improved heart/lung health doing interval training. All you need is 20 minutes per interval session (you can go longer as your conditioning improves). One way to do intervals is to sprint for 30 seconds followed by a 60 second walk or jog (you could do the same on a bike or elliptical). That would be one interval. Repeat this pattern for 20 minutes and then cooldown for 5 minutes. For those who are crunched for time, interval training is what the doctor ordered.