Junk miles, say those who believe in such things, are those miles with no specific training purpose, which therefore serve only to add to your fatigue. The name, of course, implies far worse - miles that, like the brightly coloured wrappers at the supermarket checkout, should be avoided at all costs for the good of your health. I had always assumed that junk miles were the slow, ploddy ones, but looking online, it seems definitions vary - many saying they are the miles that are actually too fast to be recovery, but too slow to develop speed.
But actually, the jury is out on whether - even purely on scientific merits - that’s actually true. The border between what lab tests or online calculators may decree as your “threshold” pace or your “easy pace” are blurred constantly - you might be tired, you might be dealing with a lot of life stress. Those things could make “easy” feel “less easy than it should” and your pace slower, even if the actual effort is the same. Equally, you might set out on what is planned as a relaxed, slow run and just feel unaccountably good, and push a little harder than you originally intended. Are either of those things bad? How can they be, if they make you feel good?