It will have absolutely NO effect on your trip unless you actually participate. The course of the 15 K starts in the Animal Kingdom parking lot at zero dark thirty, runs to/through MGM and then past Boardwalk/Beach club in to Epcot. The course closes by around 0900 when any non-athlete parkgoer would be out and about. In addition, the events drew less than 2000 people for the innaugral race this year. The 5K took place over in the Downtown Disney/Saratoga Springs area. I expect it will be a bit bigger for 2007 but nothing approaching January marathon weekend so don't worry about the presence of a few athletes affecting your trip.