Exercise: how much is too much?

I have been thinking about this a lot lately. I did not want to exercise last week because my knee hurt. My knee hurt because I gained three pounds. I gained three pounds because I had an injury (on my knee). The argument can go around in circles for eternity. It is kind of the chicken or the egg argument. What came first? What is the relation to the original question? The reason why I injured my leg was because I exercise too long, too hard and too much. So how much is too much?
When your body tells you to stop, you should listen intently. Is it really telling you to stop or are you looking for an excuse to quit? If you are feeling pain... it is telling you to stop. If you feel tired... you are looking for an excuse, unless you are fainting... which means you overdid it.
Just listen to your body and analyze the answer. Deep down inside, you know the difference. Oh, and by the way, I walked three miles yesterday and my knee does not hurt today, which means I am going walking again tomorrow.