I love the Walt Disney World Marathon Weekend! I have run the half (2016) and the full (2017), and am returning in 2018 for the full again. I love the race so much that I hope to make it an annual tradition! Disney races are seamless and a ton of fun! My favorite part about this race is that Disney is really about celebrating the fun of running and welcomes runners of all speeds.
The course is flat with great entertainment along the way - you run through parks and characters or other entertainment is set up along the way so no stretches feel boring. The marathon course goes through all of the parks and starts/finishes at Epcot. The swag is my favorite of any race I've done.
The only con that I can think of is the very early morning start at 5:30, with a required arrival time well before that to get to the start and through security. However, the early start helps to avoid any warm weather.
My biggest tip is to stay on site on a Disney property, which makes logistics and race day transportation very easy. It is awesome to be able to head back to your room after the race for a shower and a nap before hitting the parks for the rest of the day! Also, be sure to submit a proof of time to avoid placement in one of the last corrals.