Hi Elaine and welcome to the Disney Parks Moms Panel!
I am also attending D23's Destination D- Celebrating Mickey Mouse. I'm so excited and hope you are, too! I have been to several D23 Events and they are always excellent.
Disney's Magical Express can only be used on the day you are arriving and checking in at a Walt Disney World Resort Hotel. You will need to find an alternate means of transportation to your chosen resort. If you are departing from a Walt Disney World Resort Hotel, you can use Disney's Magical Express on your departure day. If you are traveling with a participating airline, I would also strongly suggest using
Resort Airline Check-In on your departure day. It's so nice to be able to check your luggage to your destination from your resort lobby.
Elaine, thank you for visiting us with your question. I hope you'll let us know if we can help you with any other planning questions!
Wishing you a magical vacation,
Amy H.