Heart rate monitors are a great way to monitor the intensity of an exercise session. Generally the higher your heart rate is, the harder you are working. This makes sense. If you are exercising hard, your muscles are producing a lot of energy and have a high energy demand. In order to get oxygen to the working muscles to help in the production of energy, the heart must pump harder and faster. More blood circulates, transporting more oxygen to cells.
Heart rate is a good indicator of intensity.
Calculating Max Heart Rate
The general guideline given for calculating your max heart rate is 220-your age. So if you are 20 years old, (220-20), your max heart rate should be around 200 beats per minute. This is a general guideline and this is some variation in this, so don't be concerned if you are exercising with a heart rate monitor and notice your max heart rate varies slightly to the calculation. The calculation is a guideline and not a hard and fast rule.
Heart Rate Zones
Now that you know your max heart rate you can calculate at what intensity you would like to work at. Using the 20 year old example again, with a max heart rate of 200bpm. You may want to exercise at 50-70 of 200, and 70 of your max heart rate may be the way to go. This zone can be set on your heart rate monitor. This does not burn the most kilojoules, and may not be the most effect method for weight loss, but that is a different debate.
Exercise over 85 cannot be sustained for very long.
So if you want to go for a long steady state run, your best bet is to program your heart rate for a zone lower than 85 is a good aerobic training zone.