" The vast majority of runners, however, seldom train at a truly comfortable intensity. Instead, they push themselves a little day after day, often without realizing it. If the typical elite runner does four easy runs for every hard run, the average recreationally competitive runner—and odds are, you’re one of them—does just one easy run for every hard run. Simply put: Running too hard too often is the single most common and detrimental mistake in the sport. "

― Matt Fitzgerald , 80/20 Running: Run Stronger and Race Faster by Training Slower