Glycogen is made and stored in the liver and is stored in all cells as a reserve to power respiration when glucose levels in the blood drop. If you have ever "bonked" or crashed during exercise it is because the glycogen reserves were depleted. Marathon runners experience this on every marathon (well mortal marathoners anyway) because the body theoretically only has enough glycogen and glucose levels to sustain about 21 miles of effort. After that the body has to start breaking down tissue to power respiration. FYI I am talking about cell respiration, not breathing.
You are correct, I have taken a few anatomy classes and never knew about muscle storing glucose. I should have went to the end all be all first (Wikipedia) before opening my mouth