This is just a brief explanation of the basics.  You need to know your actual maximum heart rate (MHR) to calculate zones correctly.  Knowing your resting heart rate (RHR) is also helpful.  The 220 minus age formula was not meant to apply to the general population and is accurate only 50% - 60% of the time.  There are ways to approximate your MHR, such as in http://www.howtobefit.com/determine-maximum-heart-rate.htm, or you could get tested at a "human performance" type lab ($$$).  Your RHR should be taken before you get out of bed in the morning, several mornings in a row, and average the readings.  Then, using the "Karvonen method", which takes RHR into account, you will usually get more realistic heart rate zones.  The formula is "Percent of (MHR minus RHR) plus RHR"  So if MHR=181 and RHR=55, 75% is 0.75(181-55)+55 which is 149.  A higher MHR moves the zones higher.  Interestingly enough, a lower RHR (somewhat desirable, since it tends to indicate a stronger heart) moves zones lower.

Hope this helps.

Len

If you run a half marathon at an 11:03 pace, your finish time will be 2:24:41.  

Seems like a reasonable goal to me, good luck!

If it helps you any, my max recorded heart rate is 205 bpm and my resting heart rate is ~52 bpm. I'm a 39 yr old female.  Last night I was doing some speed intervals with my bpm in the 190's.

I remember doing a heart rate test/experiment at 13 and discovered my heart rate was well above everyone else in my science class.  I lied about my actual heart rate because I thought everyone would think there was something wrong with me.  I also have low blood pressure, and I always thought that might factor into it - but that may not be the case.