80/20 training is a training philosophy that targets a significant portion of your training sessions (80%) to be performed at a low intensity, while the remaining 20% are at a higher intensity. This principle is widely used by endurance athletes to improve performance and reduce the risk of overtraining.
This philosophy is also sometimes referred to as the Maffetone Method as Philip Maffetone was one of the early advocates of this type of training.
The idea is that when athletes train too much at high intensities they ultimately break down and risk injury.
Like many things in sport, the 80/20 ratio was discovered through a Darwinian process where the athletes who started to train more at lower intensities started counter intuitively winning endurance races.
The good news is not only does this type of training help performance but has lower risk of injury, allows for more consistent training, provides energy for other things like family/work and there is evidence that it is better for long term health.
The outcome of low intensity training is to build a strong fat burning (vs sugar burning) metabolism with improved mitochondrial function which is the powerhouse of the body.
I have practiced this type of training for myself and have also studied it and been certified as an 80/20 endurance coach.
This philosophy is an important part of how I approach training and advising triathletes.
Further resources: 8020endurance.com , Endurance Racing and Training By Philip Maffetone