From what I've read, the "fat-burning zone" is actually 65%-85% of max HR. So, you are right where you should be. It's also good for you to reach a higher zone now and then, but mainly stay in your "working zone" or 65%-85%.
Are you working with weights too? I hope so, you don't want to do all cardio. Weights are just as important, and for women, maybe MORE important. It aids in weight loss, and strong bones.

Carole