Have you ever run a half or full marathon before? Just my own thoughts, but if you haven't tried either of the distances, I know I wouldn't start with the Goofy Challenge. There are some people who have done this, and kudos to them, but I personally wouldn't start there. I saw several people swept (not permitted to finish the race due to time limits) this past weekend who were participating in the Goofy Challenge. Disney is serious about the time limits. Several of us WISHers stayed to watch the very end of each race, and while it was amazing to see everyone finishing and the "drama" that unfolds near the end of these races, it was also a reality check to see the sweeper buses and security bikers coming through to pick up those not able to meet the requirements. It is a very real challenge, one that many seasoned runners shy away from due to the physical demands this type of weekend (and months of training) put on your body.
If you've done these distances before and know what's involved, I say go for it! It's an experience that you will never forget, as those who have just finished it can tell you. It definitely is a lot of training and preparation, but that medal makes it all worthwhile. Because as we know with these Disney races, it's all about the medals.

Make sure to register early, as it will definitely fill to capacity quickly (even though they've increase the limit to 4,000 participants for 2009, I wouldn't be surprised if it filled up by the end of February).
Hope this helps. Good luck with your decision!
Steve