If you're serious about getting the most out of your aerobic exercise, you'll want to start charting your heart rate. Using target heart rate zones to train will ensure you are training at the most effective level. There are 4 main zones to train in (resting, aerobic, anaerobic and red line); each working your body in a different way. Before you begin, you must first find your maximum heart rate in order to know the correct heart rate zones for you to work out in.
There are two ways to find your maximum heart rate: you can have a stress test done under a doctor's supervision or you can find it yourself using a heart rate monitor. As the first option may not be practical, you should consider a personal heart rate monitor. Most monitors today can be found for less than $100.
If you are unable to buy one, ask a friend if you can borrow theirs or see if your local fitness/running store or your local gym has one you can use.
Once you have a monitor, go to your local track to perform the test. Do some warm-up laps around the track for 10-12 minutes. Follow this with 4 fast laps around the track at about 80% of your maximum effort. Continue running by gradually picking up your pace in the next lap. Finally, do one more lap as fast as you can. After each 100 meters of your final lap, check your monitor watch to see what your heart rate is. You will see you heart rate increase in this final lap and the numbers that are shown will be very close to your true maximum heart rate. This number is good for about 5 years as the effects of aging decrease your maximum heart rate limit. It is a good idea to redo the test every 5 years to see if your heart rate has changed with age.
Your VO2 max is your maximum capacity to transport and use oxygen during exercise. A sports doctor and your coach can conduct a VO2 max test. These tests are extremely stressful on your respiratory system. The benefit of knowing your VO2 max is that you now have another target value, which you can use in your workouts (much like obtaining your maximum heart rate). Before you consider this option, you should consult your physician and your coach.
Note: If you do not have a track to run on, you can run on a treadmill or measure out the distance in areas around your home.