If you are looking for more info on the 2006 WDW marathon or 1/2 marathon, visit the Marathon FAQ thread on the WISH (We're Inspired to Stay Healthy Board) right here on the DIS:
http://www.disboards.com/showthread.php?t=725285
There are a lot of fellow DISer's signed up for the 1/2 marathon on Sat 1/7/06 and also a bunch for the full marathon on Sun 1/8/06.
For the person who asked about 2007-the "official" dates are not yet set, but the obvious thinking is the 1st week of Jan 2007 (5-7).
We've been there once for marathon weekend and while the area was busy on Sun morning, it was fine for us since I participated in the event. Keep in mind that in 2006 the events are split-instead of running both the 1/2 and full marathon on the same day/time, they are now being held on seperate days and how that will affect things remains to be seen.
Also-the 1/2 marathon starts at 6am on Sat, all runners should be off the course by 9:30 or so. However, the Family Fun 5K race will start right after the 1/2 finishes and when that race finishes the children's races will take place. So Epcot will be pretty busy on Sat morning/early pm for those events.
The full marathon starts at 6am on Sun, all runner should be done by 1pm or so and obviously, the later on the morning, the less it will affect your park touring unless you will be in Epcot-that is where the finish line will be and there would be more people in that area.
Good luck with your plans and hope to see some of you over on the marathon thread!
EDITED TO SAY: What I left out of my post is that the finish line is in the Epcot parking lot and while some portions of these races do go thru the parks, they also take place on the regular roads and parking lots. There may be more spectators in Epcot's lots as family members/friends lend moral support to the runners.