Find your max heart rate by subtracting your age from 220. To get an aerobic workout, work between 40 to 85 percent of your max heart rate. So take your resting heart rate do the math using the 40 percent as the low number and 85 percent as the high and work within that level.
Fifty to sixty percent of your maximum heart rate is a healthy rate for walking. Wearing a monitor is the best way to keep track of heart rate. This would be about 100 bpm for a 40 year old. There are calculators online that can help you find your bpm.
Your maximum heart rate is often calculated as 220 minus your age in years. If, for example, you are 40 years, your maximum heart rate would be 220 - 40 = 180. It is often better to exercise at 80% of this value.