Sorry to hear about the bummer diagnosis... but it's a diagnosis so now you can move forward!
Did your doctor talk to you about anti-inflammatory regimen before you leave? See if they can give you a prescription strength ibuprofen (at least 400mg) to start about 1 1/2 before you go. Keep taking it during your trip, then ween off of it after you return. (Long term usage of the anti-inflammatories was shown to be bad for your heart last year). That should at least keep your knee from flaring up while on vacation.
You will definitely have to take your time and schedule plenty of stops. Try and squeeze in some of the "lesser" attractions for a good break (ride the Liberty Belle, catch Hall of Pres, check out Universe of Energy, any of the stage shows, etc.). Those will give you time to rest and you're (hopefully) entertained in the process.
Like PP have stated, don't rule out a wheelchair or scooter. You're going to be on vacation - don't try to be a hero.