Junk miles are junk miles. That's not to say they don't have any benefit. They're just miles spent not doing what is consistant with your goal.
I rarely spend any time training any more. I did that years ago.
For reference:
Nowadays, if by myself, I generally cruise at a relatively low 110-130bpm heart rate & relatively low 175 watts average & I'll do it 12-20 hours a week. If riding with others, I'll be at about 90bpm.
I do my training on a trainer in the winter. 60-90 minutes, 5 days a week at ~250-300 watts & ~165-185bpm.
I'm probably the odd man out here, but I consider the trainer to be "junk miles" because they are actually quite hard, generally not very fun, & don't really coincide with an outdoor equivalent. The trainer is a lot like droning along on a constant 2% grade.
Miles spent outside are the quality miles & that's what I train for. The camaraderie & the skill/fitness to be prepared, to keep up with & have fresh legs for whoever I'm riding with that day...Those are the quality miles.