To improve your fitness, there are only three variables you can control and change:
Frequency–how often you exercise.
Duration–how long the individual exercise sessions are.
Intensity–how challenging the individual exercise sessions are.
Try the following: First, find your Functional Threshold Heart Rate (FTHR). There are a few ways to find your FTHR. Wear your heart rate monitor and warm up, then run hard for 30 minutes as if you were in a race. Or you may actually participate in a race that will take you about 60 minutes to complete. Your average heart rate for either run, if you were going hard, is a good predictor of your FTHR
Once you find your FTHR, determine your training zones–the heart rate ranges that will help you reach specific fitness goals–by using this table. Be aware that your FTHR and zones vary by sport. The following is how you find your zones for running. The more times you repeat your FTHR with the above methods, the more accurate your zone numbers will become.
Zone Equation To Determine Zones Zone Purpose
Zone 1 1.02 x FTHR Aerobic capacity
Source:
http://m.active.com/women/Articles/How_to_Use_a_Heart_Rate_Monitor.htm