Have you tried the Hal Higdon Walking Half Marathon schedule? It is a lot more doable than the plan that you appear to be on. If you can work your way up to the 4 mph setting on a treadmill and complete each milestone with Hal's plan on it, you will have no problem beating the threshold and will do well at the Princess. But, if you aren't prepared, it is better to sit it out. It isn't worth the injury risks. But, most of all, this isn't a race that you want to change your mind with IMO. Just the fact that it is an expensive race by itself (not including tickets, hotel, gas, airfare, etc.) if you aren't truly dedicated to giving 110%, you won't enjoy it. There is nothing wrong with moving your goal to the 2012 race. I can't stress enough how important it is to be prepared for any race. It doesn't mean that you are at the front of the pack, but your body needs to be prepared for that wear and tear. It isn't a fun walk for charity where you can take your sweet time. Disney does sweep people who don't meet that 16 min. per mile threshold. They give some leeway, but I saw quite a few people swept when I was making my way back to the hotel from the DL half. I know not everyone can finish, but I know with the money I spend going to these races, there is no other option but to finish.
So, find that motivation, join a run/walk group in your area and keep on schedule. Hal Higdon has some groups, Galloway has some groups and you may even find a local group for the Princess. That was the best thing I ever did for the Princess. Towards the end, I was getting tired, it was colder and I didn't want to freeze. But, the people in my running group pushed me out of bed, got me to keep following my goals and I finished the race. Everyone loses motivation at some point, but if you have others going through the same thing, it makes it so much easier!
Just my two cents...go through this thread. You will see so many different levels and different training methods. But, we do have one thing in common. To get that sparkly medal and finish the race!

You don't need to complete a half marathon before the race. Just get up to at least the 10 mile threshold before the race! You can do it!!