I'd drive that sucker for about 10 hrs straight and make sure it's got at least 500 extra miles on it before I took it back.
Also, I'm sure you know this, but keep a very close eye on the interest rate. When I was very young I had a similar experience with a 1989 Nissan Sentra. (Hey, I was 18) The interest rate ended at 19.9 %
I never looked at it but just the payment. I was the perfect 'sucker' back then.
But, ya, put as many miles on that car as possible before taking it back. Maybe even 'accidentaly' back into a tree.