We just did this with the
Disneyland 1/2. Spent 6 days in the parks and then I ran the half marathon on Sunday... flew out that afternoon. This was my first half marathon and I was nervous about too much time in the parks, but it ended up working fine.
I would echo the earlier advice above about hydrating and eating well. I started hydrating intentionally 3 days before the race and carb loading 2 days before. The day before I ate very little fat and not a lot of protein or dairy - plenty of pasta and rice. In the park I ordered kids meals with plain pasta and tomato sauce and ate bananas for snacks.
Saturday was our lightest day in the parks, but we were still there (and at the expo) more than 8 hours and left during the 9:30 p.m. fireworks. I was in bed by 11 and up at 3:30 for the race. I was careful not to wear my race shoes in the park. Maybe I could have run faster if we had not done parks the day before, but I didn't want to interfere too much with our family vacation and honestly, I don't think it made much difference. I was very happy with my time for my first distance race.
It was a great way to end our trip! We headed to the airport pretty soon after the race. I think sitting on a plane for 5 hours left me more sore than usual, but it was worth it.
Congratulations on deciding to go for this goal! I don't think you will regret it.