The title is misleading. A virtue is an operative habit essentially good (habitus operativus bonus). Therefore, the virtue is per se the golden mean i.e. avoidance of excess and deficit in one's behavior. "Excess in virtue" is a heroic degree of the virtue - a sure way to sanctity.
In the sheet we have virtues (but not only virtues) and vices as well. Vice is vice, not the "excess of the virtue".