Kim
You need to be registered for the Goofy Challenge in advance. They capped Goofy at 3,000 this for 2007. At packet pickup/trade show go to the Goofy booths and process your paperwork. You'll get a stylish bracelet which you get to wear until the half.
When you finish the half get the Donald medal and then go to the Goofy processing tent, where they will take your half bracelet and put a full bracelet on your wrist. This was a bit of a wait in 2006 so hopefully they'll be quicker in 2007. Wear this until the full.
When you finish the full you do the same thing, get the Mouse medal and then go to the Goofy tent. They'll take your full bracelet and hang the Goofy medal on your neck right then and there.
And then wear it with pride. People will either think you did something really amazing, or that you're just a fool. Both, of course, are correct.
Craig