Cardiovascular or cardio exercise refers to exercise that strengthens the cardiovascular system. The term Aerobic just means in the presence of oxygen, and aerobic exercise refers to exercise that you perform long enough to require oxygen. The same types of exercise satisfy both definitions, and the terms cardio and aerobic exercise are used interchangeably. Aerobic/cardio exercise uses large muscle groups (usually legs) in a rhythmic and/or repetitive fashion. Examples are running, walking, bicycling, spinning, swimming, cross-country skiing, rowing, skating, and cardio machines such as stair steppers, treadmills, and elliptical trainers.

In general while running, you should not always be pushing yourself to the absolute maximum of your fitness/cardio-respiratory capability. If you're running to lose weight or even to stay in good shape (heart heath), a good test is called the "Talk Test".

If while running you can still talk and carry on a conversation with a little difficulty (breathing), you are running at a good pace for you. If you are running so hard that you have great difficulty speaking, you should probably slow  down a little bit.  On the flip side, if carrying on a conversation is really easy, push yourself and pick up the pace a little bit more.

To lose weight or stay in good shape, you should be exercising at a pace where you are pushing yourself.  The Talk Test is a good way to check if you're giving yourself an appropriate aerobic workout. Try it the next time you go on a run with a friend, while on a treadmill, or even during an aerobics class workout.

Obesity or being grossly overweight puts a signifcant strain on your heart. Exercise and a fitness program will improve your heart health and assist in losing weight. However, some dietary adjustment is absolutely neccessary if you want to lose weight and improve your overall health.
 
If you want to lose weight, improve cardiovascular health, and lower cholesterol, you need to cut back on calories from saturated fat and sugar, eat more fiber and lean protein, and stock up on fruit and vegetables. Once you are in a soldi routine and feeling good about your health you can treat yourself once in awhile ... just stay away from indulging or binging.