Heart rate monitors are a little eer tricky to explain, but I will do my best. I went to a running camp over the summer (It was rated the best running camp in the world!:)) and they showed us the importance of heart rate monitors. I would say for most people the anaerobic threshold would be around 175. Listen up
THIS NUMBER IS THE KEY TO IMPROVEMENT
This number tells you that running within 160 – 175 bpm is conditioning your body. This means STAY BETWEEN THOSE NUMBERS ON EASY DAYS (preferably under 170).
It also tells you that 145-160 is the bpm you need to stay within on recovery. I recommend using the heart rate monitors on all easy days. For me that is 5/7 days a week(I have 2 hard days which involve sprint work, it is not needed on those days).
– Note that this may make you run slower miles at the beginning of training, but as your heart gets conditioned your mile times will improve even on your easy days. With the help of the rate monitor I am doing things I would not deem possible a couple months ago.