To me, junk miles would be a time that you SHOULD be riding a specific way but you choose not to do so and do something detrimental to your training plan. Like, you should have taken an off day or ridden Z1......but you went out KOM hunting......then the next day you couldn't hit your targets because of your poor choice. Or, you should have done those tough 3x3 intervals on Wednesday but instead decided to go ride the Zone Zero beer crawl ride instead.
Junk miles are NOT to be confused with "rider's choice" or "Z1" days. There's a reason to take time to do what you want sometimes. Mentally or physically. If you're in a training plan, just be sure to work such days into the plan so that you don't derail the plan.
If you're not in a training plan, I don't think there's a concept of "junk miles". You don't have a goal or plan, so what is there to derail or sabotage by riding a different way than normal?
If you don't formally "train", it's just poor planning if you go out and hammer around the evening before a really long group ride the next morning. That's not "junk miles". It just is what it is.