My suggestion would be to ask your doctor for advice. Your heart rate is not always just a simple measure of health or indication of proper exercise exertion. There are plenty of reasons why your heart rate may not be what you expect and none may compare to the next persons experience. For example, my own heart rate will never ever go into "the zone" due to a heart condition and the medication necessary to treat it. But that doesn't mean I won't benefit from a good workout that still keeps me under someone else's target heart rate.
As long as your doctor says you are healthy and that the heart rate you reach is safe and effective for you, then that's the advice you need to follow. She may have suggestions for you.
Good luck
