Expect it to be fun and energizing. As Len pointed out, you have lots of time to train, so take advantage of building a solid weekly base and find a training program that fits you. With regards to the event itself, the start area will be buzzing with anticipation and long lines at the porta-johns. And being Disney, there will be lots of runners, so it will be crowded. Just run a comfortable pace, don't get wigged out about passing other runners or the crowds - things will spread out after the first few miles. If you've trained really well and you can run a good pace, you'll be able to open yourself up in the latter part of the course. I'd recommend running a 10K or even a 10M event sometime before Disney just to get the feel for events that are longer than 5K. Regardless, you'll have a great time. Enjoy it!
-bill