That’s often summed up (as in the title of Matt Fitzgerald’s 2014 book on the topic) as 80-20 running: around 80 percent of your running should be easy, with the other 20 percent medium or hard.

The Pareto principle, also known as the 80-20 rule, is a universal concept that has found application in various fields, from business and economics to health and personal development. At its core ...