You'll be just fine. For me these marathons are move for fun than a race. Just have fun and don't fall below the required pace.
Totally true. I have run all the DL halfs and two WDW marathons. The environment is great for runners and walkers of every speed. For most of these I race them, so I don't get to enjoy as much as you who get to savor it more. But, last year I was injured just before the half and could not race it. So, I decided to go ahead and run it in costume (relatively slowly). It was so much fun to interact with the people more and a total blast to go in costume.
I plan to keep racing the half as many years as my body allows, but am looking forward to Goofy in 2013, where I will likely run both at a more reasonable pace and very likely do them in costume.