Large parks are naturally prone to long walking distances, so I am going to focus on parks where you have to walk long distances between their major rides. Eftelings major rides are not that far apart, but it can of course result in a lot of walking if you want to experience more attractions or dare to enter their fairytale forest.
Energylandia This is in my opinion the worst walking I had to do in any park to date. No only are 2 major rides in the far back of the park, the whole Aqualantis area is currently one big oneway with Abyssus towards the back. The Abyssus queue is quite a bit of walking, but nothing too bad. And then there is Zadra. Allthough it has been shortened by a decent bit, it is still a lot of walking. At least they open up the gate next to their lockers, so you don't have to go through the whole walk for every single ride on an empty day. But wait, theres more: Even the distance from the stations exit to the shop is a long one.
And if you want to ride Hyperion, prepare for a long walk through the whole park again. And Hyperion can also have a incredible long queue, but is usuallly shortend by a fair amount.
If you've never been there, and just to put things into a perspektive: From the park entrance (where Hyperion is pretty much next to) to Zadras station is close to a 20 minute walk, and Abyssus adds currently another 5+ minutes on top of that.
Gardaland Not only are most of its major rides a decent distance apart, but the park is located on a hilly terrain, meaning that you constantly go up and down and through weird pathes to get to your target. And then there is I Corsari. The whole ride is on the very end of a really long oneway. It's queue isn't short either, but one ride on this will easily sets you back 30 minutes until you can ride something else again.
Europa-Park Well, the park is really big and spread out. Most queues still involve some decent amount of walking, as they can't be shortened to a large extend, but this is just the icing on top.
Alton Towers Another really big park, but this really spreads out its major rides. Oblivion and Smiler, Rita and TH13TEEN (pronounced: "T-H-1-3-TEEN", and Galacitca + Nemesis are on what feels like complete opposites of the park. Once you leave one area, prepare for a lot of walking. And prepare for long queues to walk through, my highlight being Oblivion. Wicker Man is the only large scale attractions in a "central" location. Well, in other parks it would be a horror to walk to, but for Alton, it is really in the center.