Hi Andrea!
Thank you for bringing your question to the Disney Parks Moms Panel!
I just returned from Walt Disney World Marathon weekend and it was a blast! I think your daughter will love it.
The marathon doesn't sell out as fast as that weekend's challenges (Dopey Challenge and Goofy Challenge). If she for sure wants to run it, you should register as early as possible, but the marathon took a while to sell out this year (several months). The Dopey Challenge will sell out in a matter of hours because of limited spots. The Princess Half Marathon generally sells out in just a few hours, as well.
Be sure to follow runDisney on Facebook (Run Disney) and Twitter (@rundisney). They will post in advance of when races go on sale, so you can be ready.
You will find a tab with SPECTATOR information on
each event's website. It has useful information on where to watch your runner and how to get there. There will also be spectator information in the Official Event Guides that come out a few weeks ahead of each race. My favorite viewing area, as a spectator, is inside the Magic Kingdom. You can stand on Main Street, U.S.A to watch runners as they enter the park and run toward Cinderella Castle. It's magical! You don't need a park ticket to be there as it will be before the park opens for the day.
I hope this helps! Please let me know if you have any other questions about runDisney events.