Marathon weekend is pretty popular, although 2014 probably won't be quite as big as 2013 since it's not a big anniversary or anything (last year was the 20th anniversary of the Marathon, so Disney did a huge marketing push). Registration opens in early April, so I'm guessing that's when a lot of people start booking rooms. Doesn't hurt to already have a room when that rolls around. If I am remembering correctly, Disney pretty much sold out every room in its inventory that weekend.
(Everything from here down is based solely on my experience this year. Disney has been known to change how things work, but this will give you a good idea of what's going on)
As far as other things going on - there are really only 2 things you
need to do. Pick up your race packet and run the race. Packet pickup is done at the Health and Fitness Expo at ESPN Wild World of Sports. This year, packet pickup ran Thursday, Friday and Saturday. Clearly, Saturday will be too late to pick up your race packet, so you will need to do that Thursday or Friday. Disney has buses running from all Disney Hotels to ESPN, but not to/from the theme parks. You can also drive there and park for free, but it's a mad house. Both times I was down there this year the parking lot was full, overflow parking was full and they were parking people in overflow overflow parking. Basically, Tampa.
Anyway, your race packet consists of 3 things - your race bib, the commemorative long sleeve t-shirt, and a large, clear plastic bag. (The bag is for race day bag check. Put anything in it that you will want immediately after the race. A change of shoes, socks, etc.) The bib gets pinned to your shirt, and the commemorative shirt is for wearing constantly after the race, on the plane ride home, at work for the next week or so, etc.
The Expo is also where you would change your corral assignment if you want to submit a time from a 10k or longer race that your run after November 1.
Saturday is race day. If you're staying at a Disney hotel, you should be on the bus by 3-4am. If you're off-site and driving plan a little more time than that. 20,000+ people are descending upon Epcot and many of the roads on property are closed.
I wouldn't fly out on Saturday after the race. Being squeezed into a cramped airplane seat can be very bad for your legs after that much work. So if you wanted to make a very quick weekend out of it, you could fly down Friday morning and back home on Sunday.
But what's the fun in that? With this many DisDads coming down, there is bound to be something EPIC going to happen.