Well....
1. Hope you had your practice meal prior to marathon morning.
Don't use marathon morning to try new yummy things that sound like a good idea. Trust me--they aren't.
2. If your plan is to just finish--enjoy the event, but dont' get caught up with pics with the characters and mile markers until your sure that even if you crawled the rest of the way, you'd finish on time.
3. Finishing on time--you get a medal if you finish, but I can't tell you how heartbreaking it is to not get the certificate if they shut off the mats. To me--that certificate is more important than a picture with Donald along the way. So plan accordingly.
4. Potty spots--I'm trying to remember the good bathrooms to avoid the port-o-toilet. The last event I did was the Goofy. I think the half--you end up going to MK first and finish at EPCOT.
There are 2 indoor bathrooms you will (or might) pass--one is on the way through tomorrowland and the other is by the teacups. There is also one as you meander through frontier land. If you can manage your pitstops accordingly, these can save you a little bit of time and a little bit of cooties. Typically no lines as there are more potty options.
If you really have to go and time is of the essence--do not be afriad of the bushes. You aren't the first, you won't be the last and it isn't worth missing out on a medal in lieu of modesty.
5. It is too late to buy new shoes--so barring some horrific accident that renders your running shoe into a flip flop, keep with what you have. Race day is not the day to break in a new pair.
6. Have fun with the crowds--you will get some comical cheerleaders along the way...if something causes you to doubletake, pause for a second and reread the sign. Chances are that it is pretty darn hysterical as it did say what you thought it did.
7. Shop at Goodwill or any other thrift shop for throw away clothing. The marshmallow outfits they sell at expo are effective. But they do sometimes sell out. I remember one year, I didn't start the event with any extra layers as it was too hot. I remembmer one year, I bought the marshmallow outfit as all the layers I bought at goodwill were not enough. Goofy year, was just right. Had my layers and did shed them. I had to buy double that year so I had something for each day. IF it happens to be cold, you do not want to lose your energy shivering during your 1-2 hour wait before the race.
8. Busses--if on property, plan accordingly. I usually hit the first coupel of busses between 3 and 3:30. If anything it is to be on the bus that isn't stuck in race day traffic. If driving in, get there early enough, park and then nap for a couple of hours. Allow 30 minutes to get from your car or bus to the start line. It's a lengthy walk.
9. Bring money for the massage afterwards. Cash only and they can actually do some preemptive work to help your in your post-race recovery. It will not be like going to the spa--but in post-race recovery, it's all about facilitating recovery.
10. Enjoy it! Walk for a little bit when it is over. Don't stop dead in your tracks.
11. When you get to the finish line--look up and smile for the cameras so that you can get a post-race photo when they come out that shows you indeed DID it.
but you're welcome to come join us on my pretrip report - just a few others that post their progress - its been slow lately - 
They really got me thru some "tough" moments (Mile 7 in front of the Poly and then mile 12 was bad - I'm very slow 15 - 16 min miles)
and flush!! 
and I think if you arent there by 4:30 you may be out of luck - they close the roads! 
DH said he saw a few women (and many men!) jumping the fence to use the bushes (and a towel was helpful for privacy) and left over the fence... its also nice to sit on for the hour while you are waiting for the start!
