IPad with several accounts

Hallo. I have a problem with an iPad I borrowed for some time. It was an Apple ID already on the pad, with apps and all. I did not want to erase it as it was merely a loan, so I just let the apps be, and set up the pad with my own Apple ID, installed apps, even bought some. Now the loan must be delivered back and I am going to buy my own now that the Air has come. But, I need to back up my buys and the rest of the stuff. How? I would like everything associated with my account to be backed up, with the apps. Or, the entire iPad really, since there is maybe 10-20 apps that was located on it from before. But to do so I need to authorize the account which isn't mine..?