Joey - in 2011, when I did the Goofy, I left my hotel for the 1/2 at 3:50AM. Traffic was backed up on World Drive, all the way back to 192. It took close to an hour, to get into the Epcot parking lot. I headed straight for the starting line, and got caught up in a sea of humanity, slowly making the way to the corrals. I made it to my corral at 5:25. (The race always starts at 5:30, by the way. You may be thinking of your corral start time. I think E starts close to 6AM.)
The next day, I left at 3:30AM, for the full marathon, and had no problems, whatsoever.
Last year, I can't remember what time Renee picked me up at the hotel, to drive to the marathon relay. I think it was around 4:40AM? No problems at all. Drove straight in.
The full marathon usually has about half the number of participants as the half, so I think that's why we didn't encounter problems. Not sure what it's going to be like this year, since the marathon "sold out" so much earlier. No idea how many people they allowed to register, but I'm sure it's more than usual, because of the 20th anniversary.
I'm going to leave early, just to be on the safe side. I signed up with the Runner's World Challenge, because I stupidly waited until registration was closed, so the Pasta in the Park Party, Saturday night, as well as the Race Retreat, are included. I was concerned about the late hour of the party, but my training has gone to shit, the past month, so I already know my time is going to be crap. I might as well just enjoy it, and take full advantage of the race retreat, as well! I wouldn't have signed up for them, otherwise.