Using a heart rate monitor is a valuable tool in training particularly if you are training for a marathon. The key area of fitness you will be developing while training for a marathon is endurance. This can be challenging because the athlete has to slow things down from their normal faster pace to effectively develop aerobically. This means running with your heart rate rather then trying to be the fastest in your training group. Whether your goal is to run a fast marathon or finish, using a heart rate monitor is a key to smarter training.
Heart rate zone training is not as hard to figure out as you may think. In simple words you use your heart rate to help train in the correct energy system or zone you are going to race in. Training for a 10k uses a different energy source or heart rate zone then training for a marathon. The marathon requires an emphasis on aerobic endurance while the 10k requires more speed. You will need to train at the aerobic threshold. A common mistake runners make is training in a zone that has nothing to do with their goal race. In marathon training/racing it is so sub-maximal that it is very easy and tempting to go faster. But going faster is a huge mistake because if has no benefits at all. It will only make you feel wasted from the workout.
The formula to help determine your heart rate zone is 220 minus your age giving you your maximum heart rate (MHR). If you take 85% and 80% of your maximum heart rate you get your target heart rate zone. The upper limit represents 85% (MHR x 0.85) of your maximum heart rate. The lower limit represents 80% (MHR x 0.08) of your maximum heart rate. While you are training keep your heart rate monitor between these two numbers. When you train with a heart rate monitor you can set your upper and lower limits that represent your heart rate zone. The heart rate monitor will beep letting you know if you are going to fast (above your upper limit) or to slow (below your lower limit). Since you will know instantly if you are above or below your target heart rate zone you can adjust your pace accordingly.
Training within your target zone will help your body adjust to the pace you will be running for your marathon. If you start your marathon to fast the result could be walking by mile 20. I personally trained for the Philadelphia Marathon twice. The first year I ran the beginning to fast. By mile 17 I was walk/running. The next year I was in the same shape but trained with a heart rate monitor within my target zone. I wore it during the marathon race to keep me within my target zone early in the race. I ran the first 17 miles over a minute per mile slower but my final time was 35 minutes faster finishing with a personal best time of 3:17.

Author's Bio: 

Pam Daugherty has been a runner for 28 years. She has run a marathon PR of 3:17, which qualified her for the Boston Marathon. For heart rate monitors I recommend