Miles just to get in miles....not high quality. So if you run 30 miles a week and had only run 25, doing 5 miles with a 100lbs if kids and stroller just to get them in would be junk miles (to me anyway) vs miles alone where you felt better and had a better pace. Or running when you're sick or injured to make your mile goals. I have little time to run so try to do quality over quantity.
Oh. I just run to run and don't really count how many miles. I don't get lots of opportunity to run by myself since DH works long hours and travels so taking the kids in the stroller several times a week are my "real" miles. If I get off my butt and run I consider it a win. DH keeps track of my Garmin stuff but I don't.
Re: "Junk" miles