The Pareto principle (also known as the 80–20 rule, the law of the vital few, and the principle of factor sparsity) states that, for many events, roughly 80% of the effects come from 20% of the causes.Some version of this 'rule', which seems to have a rough equivalent in sectors from business to agriculture to social media (80% of your facebook interactions come from 20% of your friends), is foundational to my training methodology. In my love of complexity, i'm going to suggest that in truth the principle as applied to fitness is actually recursive and can be applied iteratively (only in terms of time invested) as one approaches their athletic goals.
In terms of fitness the principle goes something like - 80% of your fitness potential** comes from 20% of your training time. The second iteration means that 80% of that 80% comes from 20% of that 20% - creating a 64/4 principle. Now keep in mind that this really only applies in theory, and to someone following an 'ideal' training program with a proper mix of intensities. But thus applied - it is basically sound: 80% of the fitness potential realizable on that program comes from 20% of the training time. And i bet you already know what that training time is spent on - yep - high intensity work.
So as a thought experiment, lets consider that I might (barring likely injury) be able to train to run a 9-10 hour ironman triathlon if i could mentally and physically follow through on a traditional training program pedaled to elite athletes. It'd likely require about 16-20 hours of effort (on average) per week, and would (in theory) get pretty close to my physical potential. The Pareto principle suggests that by training around 3-4 hours a week and focusing on the workouts with the highest 'return on investment', i can get to about an 80% level. Apply it again and you're at about 64% of your potential on under 1 hour a week. And while 64% might seem pretty low to some folks (and it should) - i'm going to suggest that the majority of recreational 'athletes' - the ones whose goals and ambitions support the entire 'health and fitness' sector of the economy - are working at levels below this*.
*Quantification of fitness is messy as there are too many variables - but i think the basic structure of what you'd see using just about any metric would substantiate these ideas. For running endurance, you might measure percentage of potential based on marathon time for example. If I trained exclusively for a marathon and tailored my life to support my efforts, what time could i aim for? Probably somewhere in the sub 3 hour range. My idealized training would probably consume about 4-5 hours a week. Using the above logic, this would mean that I ought to be able to train for about 1 hour a week and turn in a respectable time of sub 3:45 (0.8*3 Hrs). My own experience corroborates this.
**Of course working out/training serves many purposes in peoples lives beyond working to reach their physical potential - stress relief, general health and well being, camaraderie, and just good old plain fun.