Welcome to disABILITIES.
I would suggest reading the disABILITIES FAQ (which is about the second item on the index). It gives a lot of informaiton about wheelchairs and
ECVs.
My personal opinion would be to rent an
ECV from off-site. Most people walk six to ten miles a day while at WDW, and also the hotel corridors can be VERY long.
Another suggestion is find out your state's requirements for a temporary disabled hang tag and get the paper work and have your doctor sign off on it. The tag will allow you to use any of the handicap parking areas (which will be a necessity). Also, with the policy of charging for valet parking, with a handicap hang tag the fees will be waived. Also the valets will usually volunteer to assist in loading/unloading an ECV. In Virginia I printed the required form from the DMV website. When I took the completed form to the DMV Office it took less than 15 minutes from the time I walked in until I had my tag.
If you are in a cast, you might want to have the person who puts it on you draw a line with a marker where it will be sawn off. Then bring a "Sharpie" type marker pen with you and have the Characters sign the cast. Warn them when signing to not cross the line. After the cast is removed you will have an interesting souvenier.