The 80/20 Rule of running training states that 80% of your weekly training time should be done at an easy effort level, with 20% consisting of harder running.
This lower intensity can be easily measured with the talk test; if you are able to hold a conversation during your run then you’re running at the right pace. The higher intensity needs to be high for this 20% of your training and you should be breathless and unable to talk.
Running is a predominantly aerobic activity. Your training should reflect that with plenty of slower, easy-paced running, when you are best able to use your aerobic energy systems. Marathons are 97% aerobic!
There is still benefit to training your anaerobic energy systems because you will spend some time out of breath for that hard kick at the end of a race, or powering over a sharp hill.
Training in this manner allows runners to reach their best potential on race day while avoiding injury and burnout.
