I Have Been Exercising By Running And Doing Aerobics For

I have been exercising by running and doing aerobics for over 20 years. I am a 41 yr old female. While jogging comfortably, my heart rate is usually around 168-173. While doing my step class or during a more strenuous exercise, it sometimes gets to 183. I went to a cardiologist who had me WALK on a treadmill. I think my heart rate got to 154 walking uphill for an extended time (they said since I was in shape, they had to extend the test). They said my heart looked great and responded well to exercise. But I couldn't get them to tell me whether or not it is dangerous or bad for my heart to be working out with my heart rate so high. The treadmill test heartrate didn't come close to what it is when I work out comfortably every day. So anyway, they said I could "try" a beta blocker, so I was put on a half pill of the lowest dose of metaprolol. I have been on it for a week, and I checked my heartrate today while jogging comfortably and it hasn't changed...it was 173. The beta blocker makes me feel tired. I don't like it, and I'd rather not take it if I don't have to. Can anyone tell me if the fast heart rate in itself is something I should even be concerned about? I think it's always been like this...nothing new, and it decreases rapidly when I slow down or stop exercising.