I had read on another thread that the marathon did not seem to affect the crowds in the park that badly so I think we are just going to treat it as any other day.
Mom B- if you decide to go to the parks, I would choose Epcot. We always consider not going to the parks the day we arrive but can never do it. One time we ended up buying one day passes for a half day at Epcot because we couldn't wait. Epcot is open the latest and by the time you get there (probably by 3pm by the time you get out of the airport and check into your hotel), Future World shouldn't be that crowded and you can probably do all the major attractions (Mission Space, Test Track, Wonders of Life, Honey I Shrunk the Audience, etc) there before Illuminations. Fast Passes may not still be available so if you don't mind separating on the rides use the single rider lines if the waits are crazy long. DH and I do it all the time and its amazing. A couple of times we ended up sitting together any way.
Have fun!! We leave in 10 days and I am so excited I am going to burst soon.