As a coach my benefit to an athlete is more often telling them what not to do rather than always what they must do. When it comes to running mileage, my opinion is that consistent volume is always the first priority and that a training plan needs to be based around frequency, intensity, time and type. So if higher mileage (within reason) is always good how does one define "junk miles"?
Rashelle Brown (2017) on active.com defines junk miles as "the ones you run that don't produce a specific physiological benefit." Contrary to what one might think tempo miles run too frequently are what she considers junk miles and not the slow training runs. Athletes too often have the impression that all their runs should be at a "comfortably hard" pace to improve their performance, when in fact these runs impede progression and increase the chance of injury.
Matt Fitzgerald (2013) on competitor.com advocates "a balanced approach where mileage and faster running are given equal weight." The key workouts form the core of the training with lower intensity workouts provide the basis of the mileage and these are run "without hampering your performance in your quality workouts". Too many tempo runs impact the quality sessions.
Therefore, my advice to my athletes is that you are not "logging junk miles" when you run at a slower pace to reach your weekly mileage target. These easy miles have physiological and psychological benefits that pay dividends on race day.