Disney does make some accommodations for guests with mobility challenges. This is done on a case by case basis. You can find out more information on Disney's website
HERE. In short, what you'll be looking for is a Guest Assistance Card. You can obtain this at the Guest Relations office at any of the theme parks. While a doctor's note is not required, it is sometimes helpful to the WDW cast members in determining what level of assistance you might need.
If you have issues with walking, I urge you to consider renting a wheelchair or ECV (more info is available via the above link). In addition to waiting in lines, there is a great deal of walking involved in any WDW vacation. Additionally, you might find the book
Passporter's Open Mouse for Walt Disney World to be very helpful. This guide, which is available at many bookstores and through Amazon.com, gives in-depth descriptions of every ride and attraction at WDW and details the physical challenges associated with them. It's a great resource that will make you as prepared as possible for your upcoming adventure.