I hope both of you are training. As someone who regularly does 20Ks, I see a lot of people that think they can do 13 miles without a problem. Some of them end up in the hospital.
On Topic: I haven't done the WDW races, but marathons are excrutiating. If you haven't done anything more than say, a 5 mile road race, you should probably stick to the 5k. I have been a distance runner for a while, my marathon best is just under 3 hours. But that was a cold day in Boston, which the WDW marathon will not be.
What I am saying is: If you are doing it simply because "you run through all the parks" and not because you are a distance runner, you should skip it.