If you don't have a wheelchair or
ECV and are worried about walking distance, WDW actually advises that parking in the regular lot and using the parking lot trams will be the shortest distance to walk. The trams actually drop you off with the shortest walking distance from the park. As tink32081 mentioned, the parking lot CMs do save the spots at the ends of rows for people with special need to be closer to the tram.
The Medical parking does not have any tram service and the distance from some of the farthest parking spots in that lot to the park entrance is much farther than you would walk from a 'regular' parking spot to the tram.
There are 'shuttle wheelchairs' in the Medical parking area that you can use to get to the park entrance, but there is no guarantee there will be any wheelchairs in that area when you need one. We have usually seen an empty wheelchair storage area more often than a full one.
If you park in the Medical Parking lot without a handicapped parking permit, it's important to be aware that only
some of the handicapped spots have signs designating them as handicapped parking. The majority have blue lines separating the parking spots and that is a legally defined handicapped parking spot in Florida. If you park in a spot with blue lines, you can get a ticket.
There are other spots in the same lot which are separated by white lines; you can park in those without a handicapped parking permit.
There is more information about parking in the disABILITIES FAQs thread.
One other thing - if you are staying at one of the WDW resorts, most people find it is easier to take the buses to MK since the buses drop you off right at the park entrance. If you drive, you need to get from your car to the Ticket and Transportation Center. From there you will take a boat or monorail to the MK. The monorail ramp is quite steep and there is no elevator to the monorail station, which is high in the sky.
Also, since most lines are Mainstreamed (wheelchair accessible), most attractions do not have a handicapped entrance. A GAC that allows for use of the handicapped accessible entrance will allow you to avoid stairs in the few attractions with stairs and will allow you to board the attractions with moving walkways in a place where the moving walkway can be slowed or stopped. The distance walked if using a GAC will not usually be much, if any, shorter than not using a GAC. Some lines have very long distances - for example - it is over 1/4 mile to walk from the queue entrance to the boarding area at Soarin' and is the same distance back out again.