Goals are radically different for those interested in fitness and weight loss versus competitive distance running. The worst problem with dictating a training zone is that it is often done without consideration of the duration of the exercise. To show this, in the figure below we have graphed the estimated average pulse rate a 35 year old could maintain for different race distances. It is not enough to simply look at intensity when we gauge the training stress or benefits of exercise. This should be clear from the fatigue levels felt after racing different distances, and the demonstrated effectiveness of long slow distance training for endurance events.
Intensity needs to be measured in the context of the duration of the exercise.
This is an important and different way of measuring intensity. We have used this distance based intensity idea for many years to determine the training paces of the athletes we coach. A natural extension of it is to figure out target heart rates based on the same approach. This means you should do some training that uses the energy systems you will require and some training that to make you efficient at your race pace. The use of various energy systems is affected directly by intensity.