The holodeck is too small for some of the settings it's rendering.

LOL nice joke.

I think it is supposed to have some sort of “treadmill” action to simulate large distances.

Presumably if you were "horseback" riding, the holohorse and you wouldn't be moving in relation to the ship, only to the hologround. It's all holographs and force field projections.

You could ride, walk, drive, fly for hours covering thousands of meters and when you call for the arch or cancel program, you're still in that same 50 X 50 room.

All they need is a holofleshlight. And holoschlong.

You wouldn't even need any threadmill for the 'horse illusion', all you would need is visual movement around you, and something that slowly bumps you up and down a bit. Like one of those 'bull riding machines', but a bit different.

So, one 'solid energy' is creating such a machine, and then the visual hologram renderers just move the scenery around you in a believable way, as if in a computer/video game or 'virtual reality' type thing, but without the glasses. You are just sitting in place, having the illusion of movement.

Of course adding 'acceleration' and 'deceleration' and G-forces into the mix makes it a bit complicated, but we are never told whether those actually work in holodeck or not.

I always thought there's transporter, replicator, artificial gravity and hologram tech involved. Didn't B'elanna Torres skydive in one episode with safety protocols turned off? She must "fall" in order to feel the acceleration towards the ground. Such a free fall scenario is a complicated environment to reproduce, but could be achieved using a combination of all technologies combined.

It's a TV show.

Alright, so here's how I always imagined it. All of the objects in the holodeck are projected light held in place by an energy field. This provides an environment that is indistinguishable from the real world. All the objects feel like the real thing. This would include the ground as well.

Since the ground is a projected object, just like anything else in the scene, it doesn't have to remain static in relation to the physical room. As you walk in any given direction, the ground/floor moves beneath you keeping you centered in the room.

This is a treadmill of sorts, but not as though the physical floor of the room is moving, but rather the virtual floor/ground created by the holodeck.