Hi and welcome to the disABILITIES Board. I see this is your first post to the DIS boards, so I want to give you a special welcome.
I think you are perfectly right about needing an
ecv or wheelchair. Most people don't realize how far you walk at WDW. The distance around the waterway at WS is around 1.3 miles (about 2.09 km).
You can rent a wheelchair or ecv at the parks, but there is no guarantee that they will have one when you need it.
Here's a link to the DIS page about touring with mobility disabilities. It does include information about renting from the parks.
A lot of people choose to rent from one of the off-site companies. That way, you also have it available at your resort and if you go someplace like DD. Here's a list of the places DIS posters have reported renting from:
Care Medical:
http://www.caremedicalequipment.com/
Walker Mobility:
1-888-726-6837
www.walkermobility.com
RANDY'S Mobility is in Kissimmee 407-892-4777
http://randysmobility.com/
Colonial Medical
http://www.colonialmed.com/about_cms.html
We haven't rented because DD has her own wheelchair, but a lost of this board's regular readers can help out with questions about renting.
And your last question: MK and Epcot have some ways to get from point A to point B without walking.
MK has a train that stops at the entrance of Main Street, Frontierland and Mickey's Toontown Fair. It is a good way to get far into the park (or back out again) without walking. There are also horse drawn trolleys that go from the front of Main Street to the castle and back.
At Epcot, you can ride the Friendship boats across WS lagoon.