Rating of Perceived Exertion, or RPE, is one way of measuring the intensity of a workout. If you do not use a heart rate monitor, then it is the method to determine what intensity zone you are in, and where you need to be.
The Borg Rating of Perceived Exertion is a 1 to 20 scale; for our purposes, we will focus on the 6 to 20 range. The range was developed because it represents about 10% of the heart rate experienced by a moderately fit 20-something person. In other words, 6 = 60 bpm, or roughly a resting heart rate (no exertion), and 20 = 200 bpm, or maximum heart rate.
Most training plans are based on Intensity. Intensity is described by five training zones, Zone 1 to Zone 5. Many triathlon training plans divide the top zone in three: Zone 5a, 5b, and 5c. Athletes can use these zones to "tune" each workout for the best results.
The zones are set based on percentages of generally one of three values:
Obviously, the best way to get your maximum heart rate, and thereby your heart rate training zones, is through a treadmill or bike trainer test. Some health clubs offer this service, as do many university and college athletic departments. However, you can come up with a fairly good estimation of your training zones by using the Karvonen Method:
Mark Allen is well known as one of the "Big 4" of Triathlon’s early days, and won the Ironman World Championship (a.k.a. Hawaii Ironman) six times. An early proponent of training by heart rate, his somewhat unique training zone calculation is used by many athletes.