As you can see by my sig, we went the second week. The Christmas decorations were up and Christmas music was played the entire week which was a tad odd. We did miss the Osbourne lights by a day, which was sad, and the day we flew in was the last day for the resort Christmas trees, as ours was gone the next morning, though the park trees remained.
And everything was walk-on! It was fantastic and I really recommend going then if you can.
They day of the marathon was fine. We were in Epcot that day and all they ask you to do is kind of congregate at a few "crosswalks" and dash in between the marathoners when there is a break that way the race isn't interrupted. We waited maybe a minute, darted across, and found the area near Mission: Space and Test Track to be even more deserted than earlier that week because people couldn't be bothered to go over there!
If you're driving around or staying in a resort with a lot of runners it might be more trafficky around the marathon, but if you're getting into the parks around 9, 9:30 then it's already underway and spread out by then so I wouldn't think you'd notice a thing.
