Also, if you bought your ticket through United's web site, at the bottom of your confirmation email it says quite clearly what the checked bag fees are and what the applicable weight limit is.

There is little you can do about this now, but I recommend you go to Amazon and buy a handheld luggage scale. They cost about $8. I bought one a while ago and its super helpful to make sure I stay within the limits, particularly when I travel with my golf clubs and other weird/potentially heavy things.