Raggedy, try to relax. Your knee condition does not mean the end of your vacation enjoyment. You do not need to be stuck at the resort alone.
Sue is correct - there is more walking around that one initially realizes.
I have a knee condition, so I rent an
ECV while on vacation at WDW. If you choose to rent an ECV, then I would definitely rent an ECV from an off site supplier. I have only rented from Walker. My experiences have been very good. The ECV has been delivered in a timely manner; the battery has lasted all day & night; the cost is similar to Disney & I have the ECV more than just in the parks; etc. Walker provides a battery charger that I use at night.
I started using an ECV at WDW by renting one at each park. I rented from Disney for a few trips & learned over time that renting from Disney presented a few "issues:"
*no guarantee the park will have an ECV regardless of time of day;
*if you want to park hop there is no guarantee of an ECV at the second park;
*four wheels on a Disney ECV vs. three on off site one (at least the ones I have rented from Walker); three wheels give me a better turning radius;
*battery holds up a whole lot better on the rented ECV vs. the Disney one; I can tell you where in each of the four parks where my Disney ECV "died;"
*if you rent an ECV you will have it every where you want it - the parks, Downtown Disney, other resorts, etc.
Try to stop worrying, have fun & enjoy your trip!!