My iPhone 4s battery seems shot. What can I do?

I'm not entirely sure when it got so much worse but my iPhone 4s (which is now about 18months old) will run out of charge after around 6-8 hours. This is not with heavy use - maybe some email checking, text messages, a call here and there. Sometimes I have to make an early detour home to charge before going to the gym. I'm assuming the battery needs replacing and wonder if an Apple Store would accept that and maybe swapping the phone or whether I'd likely be without it for sometime? I'm also assuming it's out of warranty. I don't want to pay the cost for an early upgrade at what is likely mid-way product cycle.