I just had to comment on this. When I was first contemplating doing Goofy, people on here would say, "If you can do a marathon - you can do Goofy. It's really only marginally harder to do Goofy than a marathon." I thought they were all big liars. Turns out, they were right. If you are doing a good training plan for a marathon and add in a few back to back training runs so you get a feel for running on tired legs, you'd be surprised to find it's really not that much more difficult than the marathon alone. It's just those early morning wake-ups!!
Just wanted to chime in on the already good responses you have. My biggest advice is to play with the intervals. I firmly believe that everyone has a "sweet spot" on their intervals where their pace is the best and they feel the best running it. The Magic Mile is a great way to get an idea where you should be, but don't be afraid to mix it up. I found that when I decreased my intervals, my overall pace actually got faster. Seemed counter-intuitive, but I was getting too fatigued on the run portion and then not recovering enough on the walk, so I would walk slower and slower as the miles added up and that drove down my overall pace.
