You can burn more fat and calories by intensifying your workout so that your heart rate climbs over the 70 percent mark. It's not a good idea to do this in the first several months of an exercise program, however.
The added strain may cause you to become discouraged or burned out on exercise. In addition, pushing yourself too hard means an increased risk of injury. If you have a history of heart or coronary artery disease, or if you are at risk for either, talk to your doctor before starting an exercise program.
I recommend a target heart rate of 50 percent of your MHR if you have not been exercising regularly. After a few weeks, you can gradually intensify your workout until you are reaching the 60 to 70 percent of MHR range