Hi cgdmom,
Since Disney accepts about 16,000 for the full marathon and another 6,000 for the half, you should find the registration process to be a breeze compared to getting PS reservations at Cindy's!

. Last year, I think it took at least a few weeks after registration began before the full and half marathon filled up completely. (The Disney web site provided regular updates regarding how close they were to reaching capacity, which is really helpful for those who are still deciding if they will participate. I'm also pretty sure that the half filled up quicker than the full.) That being said, last year I registered on the first day and probably will do so again this year...I just wouldn't worry about staying up late or waking up early to register.
I'm pretty sure that the next WDW Marathon is scheduled January 11, 2004 and that on-line registration (at the disney.com website) begins on March 1, 2003.
January 2003 was my first WDW Marathon (my first marathon anywhere, actually) and it was wonderful fun. The marathon was very well organized in terms of such things as food, water, and medical stations. The volunteers were amazing and there was a surprising amount of crowd support. You need to get up quite early to get to the staging area by the time that Disney requires but, in my case, I was so excited that it really didn't matter. If you are staying at a Disney resort, I think they tell you to take a bus by 4am...so make sure that you have appropriately warm dress for all the waiting around time. (In my case, I woke up at 3:15 am, reached the staging area at about 4am for the 6am start.)
We mostly visited the Universal parks that weekend, where the crowds were very light. However, we did visit MGM on the day after the Marathon and all the rides were virtually walk-ons.
Hope this helps!