Ellie:
I've been to Disney for the past two
marathon weekends and found the parks to have low crowds, not only because the runners are busy, well, running on Friday, Saturday and/or Sunday mornings but also because their families are either watching them along the route or meeting them at the finish line.
If you use Disney transportation, you will see little disruption except for heavier wait times on Sunday for the monorail. Disney will post lighted traffic signs warning of road closings the night before each morning run event, but it should only affect you if you are taking a car to the Magic Kingdom or Epcot for rope drop. And even then, the roads are clearly marked based on my past experience. Although the half-marathon route takes runners through Epcot and Magic Kingdom, the bulk of runners are through the Magic Kingdom before official park opening. Marathoners will run through all four parks, but with very little disruption--but you may want to cheer for them if you're in
Epcot where the finish line is located.
Wishing y'all a magical vacation!
Whitney