The idea of aerobic exercise is to fool your heart into thinking you are active all the time. Then your metabolism will operate at a faster rate 24 hours a day. You'll have more energy, sleep better, eat sensibly, lose weight, and feel more optimistic due to increased oxygen uptake and to the fact that you are taking your health into your own hands. The way you do this is by operating your heart at a faster speed for a minimum of 20 minutes three times a week. On the day after a longer run or greater exertion, always take it easy.
Condition is improved by stressing, then resting the muscles.
Do not go over 10 beats per minute more than that number, and try not to drop more than 10 beats below it. You can also measure your pulse in the mornings before you get out of bed. As your heart gets stronger, your resting pulse will get slower. You know your body better than anyone else. It may have let you down in the past, or you it, but it is going to do wonderful things for you in the near future if you keep training and treat it with respect.