I am a 46 year female. I have been running and working out regularly for about 8 years. I have completed several half marathons and one full marathon. I am interested in training for another full marathon, but I have a problem with my heart rate. I am having a hard time keeping it below 170-175. When it gets that high, I have a hard time continuing to run. I have to stop and rest. I am not really running at a fast pace. Probably around 9:00 minute miles. I recover very quickly and then I can go again. It is really frustrating, because I consider myself very fit. I am 5'5" and I weigh 125 lbs. What can I do to train my body to control my heart rate?