My experience:
Before my first full (Houston 2011), I did not regularly run to train. I did lift and cardio frequently and was in pretty good shape otherwise (for example, I set a PR--that still stands--in the half 6-8 weeks before), but I was nowhere near ready for peak performance in a full. About 1.5 weeks out, I did my longest run of 16 miles. I needed that run to give me some level of confidence going in (that I could at least make it that far).
My thoughts:
Running a full with as little training as I had would definitely not be something I'd recommend. I kept a good pace for the first 18 miles or so, but the end was very difficult, and my legs could not handle the pounding (breathing and energy were fine, but my legs cramped up and the like). In fact, I did not run again for a while, and didn't run another race for 10 months.
This year, I trained with a running group, and it made a HUGE difference going through an entire training program (I cut my full time from a 4:10 to a 3:41). I would recommend holding off and doing a full program as my primary recommendation.
On your original question, I would say this though--IF you are 100% dead set on running the Austin full, and you haven't had any longer distance runs (16-18+ miles), I would certainly want to have at least 1 run of that distance prior to attempting the full. I'd do it 1.5 to 2 weeks out though--don't wait until the week before. BUT, I really don't think that's a great idea. I would NEVER run another marathon with as little training as I had then.
[This message has been edited by thann07 (edited 2/4/2013 2:51p).]