A big advantage to trying one of the
ecvs at places at Target or Walmart is that they are very large and cumbersome (and also have a cart stuck on the front). Anyone who has driven one of those will find a rental one tiny and easy to drive in comparison.
It is a good idea to practice a little before venturing out. Get used to backing up and try a little parallel parking before you try to get on the bus the first time. That way, you will be more comfortable and not so worried about what you are doing.
I would suggest Epcot is a good park for a first time ecv driver. The pathways are wide, so it's not so crowded feeling. I would save MK until you feel confident. Because it is a smaller park and things are close together, it gets very congested in some spots. THere are also more kids to be watching for at MK.
As Anne mentioned, the CMs will show you where to park if you want to walk into rides/attractions. It's usually by the strollers. If you need to avoid things like moving walkways that are used for boarding, you will need to bring the ecv in line with you (or go to Guest Services and explain what your needs are so they can make suggestions, like possibly getting a Guest Assistance Card). Some attractions have a long path to go from the queue entry point to actually getting into the ride - 2 I can think of are
It's Great to be a Bug and
Kilomanjari Safari (which is not recommended for people with back or neck issues).
But, ecvs are pretty easy to drive. My FIL finally rented one this year and had no trouble at all.