Hi Katherine! A mini-vacation during the Princess Half Marathon sounds like a perfect mother/daughter trip!
Obviously, the longer you can stay is the better, but I would recommend at least four non-race days. Ideally, you could get there on Friday evening. You could go to the expo on Saturday to shop and pick up your race packets, eat a good meal, and get in bed early for Sunday. The race would be Sunday morning and you would have the rest of Sunday to explore the parks or the resorts. (maybe a spa day??) The next days could be spent at the parks and having a magical time! Be sure to wear your medals around! I prefer to take my vacation AFTER a race. To me, it is nice to have the race under my belt so there aren't any nerves or early bedtimes when I want to be at the parks all day and night!
My favorite resorts are the Epcot Area Resorts. I love being able to walk into Epcot and ferry over to Hollywood Studioes. These include the Boardwalk, Beach Club, and Yacht Club.
The Grand Floridian would also be a great mother/daughter Deluxe Resort. It sits on the monorail and is very elegant. There are some great dining choices there and a newly renovated spa, called
Senses.
Make sure that whatever resort you choose is a host resort. They will be published on the
Disney World Travel page for the Princess Half. Last year, all of those resorts were host resorts, and probably will be again, but definitely check before booking. Staying at a host resort with the runDisney buses is very beneficial for traveling to all race weekend events.
I hope this helps! Please let me know if you have any other questions for your magical trip!