Hi Katie! Congratulations on signing up for the 20th anniversary of the Disney World marathon! It's going to be so amazing!
Your family will not need tickets to cheer for you along the course. They can even go inside the Magic Kingdom to cheer you on as you head up Main Street U.S.A. toward Cinderella Castle. That is around mile 5 and all runners and spectators will be cleared before the park opens. If they plan to be inside any other parks during the run, they would need tickets for that. The finish line will be in the Epcot parking lot.
There is a great spectators tool on the
Disney World marathon website. Just use that link and click on the "SPECTATORS" tab to get more information about viewing areas and transportation.
As for visiting parks after the race-- you would all need theme park tickets to go to any of the parks. You can get a Magic Your Way ticket with your resort package by calling 407-WDW-IRUN, or you can purchase park tickets anywhere they are sold.
Please let me know if you have any other questions! Good luck with your training!