Remember this: cardio exercise doesn't have to be aerobic! Aerobic exercise is an activity that is done continuously for more than 2 straight minutes (like treadmill and biking). If you want to tone up your body and look more athletic (but not too bulky), you should perform the majority of your cardio exercise using sprint intervals. Doing sprint intervals will activate your bulky fast-twitch muscle fibers to give you a more lean and toned body . Just take a look at an athlete who competes in a speed sport. Their bodies are "ripped" for a reason (most of their training is fast and explosive). Research has proven that the heart-health benefits of anaerobic exercise is superior to aerobic exercise.
Sprint intervals work this way:
Sprint with maximum effort for 1 minute and walk or jog for 2 minutes to recover. Do this for 20 minutes. You could start by running on a flat surface and progress to inclines. This type of cardio should only be done 2-3 days a week to avoid burnout and injuries.
A lower level of cardio intervals could be done this way:
Run at 75% of your maximum heart rate for 1 minute and then jog for 2 minutes to recover. Complete this cycle for 20 minutes increasing the intensity each time up to 90%. Intensity can also be increased by using an incline on the treadmill or increasing the resistance on the bike. You should vary your intervals each workout to keep your body guessing.