The virtue of moderation is also known as temperance. It is the strength to resist attraction to seemingly good things. I say “seemingly” because whenever we overdo something we run the risk of turning something good into something bad.
We have all experienced this eating our favorite foods; we eat too much and we regret it later. In sports, it is possible to work-out too hard. We might have had the courage to get off the couch to train, but our bodies need time to rest. If we are always working out we run the risk of wearing ourselves out rather than building ourselves up. So too with using performance enhancing chemicals, which may produce good short term benefits but cause significant harm later on.
Another example of over-doing it would be playing so hard that we actually start fighting. We’ve all seen guys who keep playing after the whistle blows. A good player who is a good person plays hard, but when the play is over they stop. Otherwise they will get their team penalized and undo all the good they have done up to that point. In short, moderation is self-control. We put limits on our behavior for a higher good.