Before you try using iTunes, try closing all of the apps on the iPad and reset the device. then try using the WiFi udpate again. WiFi is so much faster if you can get it to work.
In order to close apps in iOS 7, you have to drag the app up from the multitasking display. Double tap the home button and you will see apps lined up going left to right across the screen. Swipe to get to the app that you want to close and then swipe "up" on the app preview thumbnail to close it.
Reset the iPad by holding down on the sleep and home buttons at the same time for about 10-15 seconds until the Apple Logo appears - ignore the red slider if it appears on the screen - let go of the buttons. Let the iPad start up.
Try the WiFi update one more time.