Did you condition the battery yet? You need to do this if not. Use the machine normally, but ignore the low battery warnings, just hit OK and keep computing until it totally runs out of juice and goes "dead" (it's not really dead). Then plug it in and leave it plugged in overnight. Don't unplug it for several hours. It needs to fully discharge and fully recharge without interruption. IIRC, Apple recommends you just leave the lid closed while it's recharging this way for the first time. Next day (or whatever), use it as you normally would, being sure to give it a full discharge and recharge at least 1x a month.
You'll also get a longer runtime if you dim your display, turn off BT, etc. Might want to check your Apple Menu--Sys Prefs-->Energy Saver and make sure the box for "Put hard disk to sleep when possible" is checked.