Aim for 50%-85% of your target heart rate while you exercise. To calculate your target, subtract your age from 220. The number you get is your maximum heart rate. Depending on your fitness level, your target heart rate will range from 50 to 85 percent of your maximum. For example, for a 55-year-old, the equation would be: 220 - 55 (years) = 165: the maximum target heart rate. 50% (or half) of 165 = 83 beats per minute. 85% of 165 = 140 beats per minute. So for a 55-year-old, the target heart rate is 83 to 140 beats per minute.
Ask your health care or fitness team whether interval training, in which you vary the intensity of your workout, would be right for you. For instance, if you're running or walking briskly, you could pick up your pace for a minute or so and then return to your normal pace for up to 10 minutes -- and then speed up again.