The only miles I've ever called junk miles were the last few of a long charity ride when I'm just coasting back to the starting point. I may have at some point, while trying to keep a friend moving that was running out of gas on a metric century referred to the last 5 miles as garbage miles anyone can do. He agreed and kept pedaling.
Hell, I rode for about an hour yesterday, making a few stops back home to make adjustments to my new bike. I even counted that as good ride. Worked up a sweat and got winded somewhat. Now I'm ready to get a couple of good hours on the new bike in comfort.