Every marathon training program relies on mileage. You have to put in the distance to prepare for the race. But, after you finish that first one and prove you can do it, the question gets a bit murkier. How do you maximize your workouts to run it faster the next time? At this point, you are likely to run into the "garbage miles" debate.
Garbage miles (also known as "junk miles" and "trash miles") are notoriously ill-defined but they usually refer to mileage that is needless or unnecessary for one's training. The pejorative nomenclature is revealing (the opposite term is "quality miles"), people's definitions of the term tend to vary quite a bit but you better believe they feel strongly about it.
Some folks use the expression to describe miles that shouldn't be included in your weekly mileage totals. Like, say, parking at the far end of the lot at the supermarket and jogging from and to your car. This can turn into an ugly debate on whether warm up and cool down miles should be included.
Others see the term as describing low-intensity workouts in general and, boy howdy, is there some disagreement over those among the runnerati. That's further complicated by the conflicting science on the matter.
Some studies have found that athletes who have emphasized high-volume, low-intensity training with a focus on aerobic conditioning outperform those that loaded up on the high-intensity efforts. Meanwhile, other studies have argued that high-intensity workouts maximize mitochondrial density which could be a key to endurance performance. The result is message board chaos.
And, lastly, there is a group that simply uses the term to describe workouts that aren't geared toward training at all and argues these are actually essential for your mental and emotional well being. The negative terminology has been transformed into irony: garbage miles are for taking the time to smell the roses.