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.
Find Maximum Aerobic Heart Rate
Start with 180 – your age
Adjust for your fitness:
This number is your Maximum Aerobic Heart Rate (MAHR). For endurance workouts, train in a zone that is from 20 bpm below this rate up to it. This will build cardiorespiratory health without overly taxing your system.
For example, for a 40 year old who is starting a workout program: 180 – 40 = 140, minus 5 for not exercising regularly, equals a Maximum Aerobic Heart Rate of 135 beats per minute. For training, workout from 115 to 135 bpm.
So what about speed work? Go for 15 to 20 minutes fast, above your MAHR. This time can be continuous, or broken into interval segments of one to five minutes. If doing intervals,
You may find these HR zone to be very low, but remember Mark Allen has been very successful using his own technique!