I've not stayed at this Residence Inn, but have stayed at the RI at Harbor and Chapman. I believe the breakfast buffet is the same. It has hot dishes, including some do it yourself waffle makers, in addition to the usual continental breakfast fare. One of the better complimentary breakfasts, if you ask me. FWIW, I've never had a bad room at a RI.
As far as walking goes, it can be done, but it would not be a quick walk like some of the motels right across the parks on Harbor Blvd.
Attendance shouldn't be bad in early December. I'd guess most of the schools won't let out till the week before Christmas.
Congrats on your upcoming silver anniversary. If you haven't been there for almost 25 years, you'll be amazed at the changes. At least, with the addition of DCA and DTD.