Hi Melissa!
Thank you for bringing your question to the Disney Parks Moms Panel!
My husband and I ran our first marathon at Walt Disney World in 2013 and it was such a great experience!
It's hard to suggest an itinerary unless I know if you're planning to visit the parks before or after the race. If you're going before, you would want to pace yourself a little more, eat properly with a marathon looming and get in bed at a decent time. All of that would be out the window if you're visiting the parks after your race!
Basically, if we had five days, I would plan a full day at Magic Kingdom, Epcot and Animal Kingdom with a half day at Hollywood Studios. Then, use your last day to re-visit some of your favorites.
We love the parks at night, so after a full day somewhere we will wander into a different park that evening. It's a family tradition to hop back into Epcot in the evening to get a pastry at
Les Halles Boulangerie-Patisserie in the France Pavilion. The Magic Kingdom is also extra magical at night. On our most recent trip, my boys and I set the goal to be the very last guests to leave, and we did it! It was pretty cool to be (mostly) alone on Main Street U.S.A. A snack from the
Main Street Bakery and a seat on the ground for nighttime parades sounds perfect!
I hope this helps! If you want more specific recommendations, please let me know if you're wanting to play in the parks before or after your race!