
I agree with the other posters and made a quick picture to show you.
The Harry Potter area in Universal Studio is circled in red and the route you would take to get there is also in red.
I marked the path of the train between London/Diagon Alley and Hogsmead in green and circled Hogsmead in green.
Parking ramps and drop off lots are across the street from Universal City Walk.
There is a very long skyway that goes between the parking ramps and City Walk. There are some moving sidewalks in the skyway, but it's still a long way to even get to City Walk, then you need to cross that to even get into either Islands of Adventure (Hogsmead/Hogwarts) or Universal (London/Diagon Alley).
You will walk at least a 1-1.5 miles just to get from parking to London according to Google maps. That surprised me because it feels longer!
Once you are in London/Diagon Alley, there are a lot of little streets and areas to explore, so you could walk another mile or 2 just exploring that area. Even though it looks small on the map, there are a lot of little streets,most more walking than it appears.
As others mentioned, you would probably need to park the
ECV outside of shops - they are pretty tight.
The Hogwarts Express train goes directly between London and Hogwarts and does accommodate
ECVs, so you will be able to get between the 2 Harry Potter areas without walking. Without an ECV, you could be standing/walking in the queue for the train for 30 minutes or more.
There are fewer shops in Hogsmead - there look like a lot more, but the majority are just storefronts where you can look in the windows. The ones that are open shops are very tight - it's hard even navigating to walk through.
Universal doesn't allow ECVs in most lines, so it makes sense to drive between places since you will be walking (or riding in an attraction wheelchair) in lines.