Hello! Congratulations on completing the Disney World marathon! That is great!
I understand that the races start really early! This is for several reasons, but mainly because they have to get roads and traffic back to normal at a decent time for everyone that is there to explore the parks.
Normally, my family will sleep in and be there to catch me at the finish line. Of course, if you stay at a host resort for the race, your family can use the buses to get to the race area. It wouldn't be too awfully early if they wanted to see you come through Magic Kingdom at around mile 10. They could line up on Main Street and watch you run toward Cinderella Castle. They wouldn't need a theme park ticket to do that, and they could take the monorail there from the Epcot parking lot (where the buses would drop them off).
You may also check out the
chEAR squad packages that are available to purchase. They offer things like priority seating (in front of the castle and at the finish) and also use of the Race Retreat tent while they wait for you to cross the finish line.
I hope this helps answer your question! Please let me know if you have any further concerns!