They do shut down some of the roads. When I ran last year, I remember there being traffic backups on several of the roads when we were running on the other side. The race starts at 6AM, and since your flight is at 11am, I'm guessing that you would typically leave between 8:30-9am. I would definitely allow for extra time that morning. If you are taking Magical Express, I would assume that they would schedule in extra time to allow for traffic delays. Better safe than sorry (although missing your flight home is much more tolerable than missing your flight to WDW!...there's pretty good shopping at MCO!).