Particularly this: The Landcruiser from the game, FAR: Lone sails.

Close up of the smaller vehicle used by the player in the Game:

So how would you incorporate the existence of a machine similar to this into the setting?

according to whatever scrap of lore there is in the game, this thing and the tiny vehicle the main character rides/lives in are called "oppidumotives". vehicles designed to traverse the post apocalyptic world the game is set in. Their form of fuel source is some kind of liquid energy created through the burning flammable objects.

also it's in cutaway format, so this isn't what the exterior looks like.

I mean imagine seeing this thing wandering aimlessly across the wasteland and various denizens thinking it's a giant monster. especially after it unknowingly tramples over several settlement. It it's pretty stuck in autopilot as it's crew are either dead or abandoned it long ago and is the tomb of the pony that designed it.

I'd say the characters meet some one who hires them because they either found the plans for the thing so knlw about it, or figured out its not a monster and wants to board it and disable it.

Few chapters spent exploring the abandoned machine.

The guy that hired them may be a good guy, some one willing to stop the threat, wants to study it or a bad guy.

Be one of those things need to decide at the time either way the main use would be exploring it and using it for some prewar world building.

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A faction could also hire them since this thing looks like it would be perfect for use as a mobile base.

However, I don't think it would have been used by the military, at least not yet. Like this was a prototype for a projected series of walker type vehicles that never came into fruition due to the war. Making it the only one of it's kind ever built.

the smaller vehicle could have been something built post war, being based on the technology of the larger machine but improvised and at a smaller scale due to limited resources.

I personally get the vibe that the Rich family might have some sort of connection to the origins of this machine.

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My first thoughts are to consider what terrain it could operate in, and why people would have made it.

For terrain, it's a very large vehicle, with proportionally small legs. It's going to be restricted to fairly open terrain that isn't too rough. Deserts, plains, and open badlands would be ideal. Gentle hills are probably okay. It's tall enough that it could probably move through light forests, but forests also tend to have a lot of rough terrain that could be difficult for a walker like this (or even dangerous if driven through incautiously). It'll handle uncleared woods somewhat better than wheeled or tracked vehicles, for the tradeoff of being unable to use regular roads to pass through more easily, and lower speed in open terrain or on roads.

So what's its purpose? Well, it's big enough to serve as a mobile base, so the first thought that comes to mind is a long-range expedition vehicle for exploring harsh environments, with terrain that is mostly open but with enough rough patches that would restrict wheeled or tracked vehicles, but not too rough (no mountains, for example). Deep deserts, barren wastelands, featureless tundra, rolling badlands, etc. Which means it'd serve wonderfully as a Wasteland survival base. It would presumably have all the facilities needed for long-term survival in harsh environments, and might even be environmentally sealed. Post-war, that seems like a great vehicle for exploring some horrendously irradiated/contaminated areas of the Wasteland. But it's also a huge machine that's going to have a huge amount of maintenance needs, so this definitely seems like a "major faction" kind of thing. That or fancy arcanotech self-repair, like the Steel Ranger armor.

Another possibility is that it was specifically created with a post-megaspell environment in mind. Maybe it was intended as a kind of mobile Stable, able to protect its occupants from radiation while moving slowly around the landscape to salvage necessary supplies. This is maybe a little eccentric, but it wouldn't be the weirdest thing ponies did.

Finally, it could have a military purpose. Maybe it was intended as a mobile base of operations for ground forces on remote fronts. It's probably slow and looks to be unarmed, so it's probably not good as a breakthrough unit, but it could help establish a front in terrain that would otherwise make supply a serious issue.

All of these come with the big caveat that there's basically nothing it could do that a cloudship couldn't do faster and better, which kind of raises the question of why it was created in the first place. One possible answer is that whatever group built this thing either didn't want to rely on the pegasi (perhaps because of growing tensions), or couldn't rely on the pegasi (such as the Ministry of Technology refusing to divert crucial combat cloudship production for non-combat tasks). Or it might be a non-pony design by a kingdom that didn't have any equivalent to cloudships. Or there's even the possibility that some post-war group made it as a mobile settlement (like the mobile Stable example), though that's a tremendous amount of resources and construction needed.

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Raider Magnets.

I can see something like that being commonplace in a region of the world that has recovered sufficiently and developed the far-reaching resource infrastructure and cooperation necessary to have the raw materials and energy sources.

The Enclave has its Thunderhead warships, after all, which, IIRC, are held aloft by clouds anchored to their hulls somehow. Exactly what powers them is never described, but just as compact fusion power sources were developed before the culmination of the resource wars in Fallout, so, too, might Equestria, or its remnants, develop power sources (spark batteries/generators and such) that would surpass the coal-fired power plants upon which they had depended.

In my own fic (To Bellenast), the city of Bellenast and its surrounding kingdom depend primarily on hydroelectric and wind power, and solar power in some of the areas beyond the cloud curtain; howsoever, the kingdom doesn't have a huge range of transportation options, due to lack of resources and limiting terrain. River barges and ferries are popular. They have some rail lines, but relatively few trains to occupy them.

Large exploratory or passenger aircraft would take vast amounts of resources to build and maintain, particularly if they're the self-levitating kind, rather than gas-envelope airships. If you take a realistic approach to that sort of world building, then you need first the population, then the demand, then the materials, and finally the fuel. My view on the demand part is such that, in the absence of a national network of fuel supply, ponies simply would rely on pulling their cargo over ground or along rivers, because making and supplying an engine is so much more complex than building a wagon and harness.

All that said, walking vehicles are hilariously impractical, mechanically complex, and inefficient when compared to literally any other type of ground transport. Road wheels, tracks, rails, or even sleds pulled over the dirt are all better than the nightmare of legs. Scaling the vehicle up only makes that worse. There's too much ground pressure for anything like that to work, just to start. Walking vehicles exist for the Rule of Cool alone.

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Well, if I needed to explain why the hell something like that even exists in the first place I would blame MWT. Basically Landcruiser was, on paper, some sort of logistic vehicle/mobile base. In practice it was a dud, non viable project intended to siphon money from MWT and nothing else. Then ponies who were planning that thing get busted by MoM, general bureaucracy doesn't catch up and a couple of prototypes are produced, before project is recognized as failure and shelved to some warehouse, where (insert whoever is intended to have it) finds and claims that thing centuries later.

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about the thing you said about prototypes, the Landcruiser/oppidumotive in the game featured a private office/cabin that serves as a tomb for it's designer. there's a series of photos in there that show the process of that's machine's development that shows at least two prototypes made before, one identified as the dkomotive and the okomotive that the protagonist of the game uses.

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or a mobile base used by a chapter of the Steel Rangers

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the Landcruiser/oppidumotive in the game featured a private office/cabin that serves as a tomb for it's designer.

Well, if we are going with it being wartime prototype idea with tomb of it's creator within should be discarded for obvious reasons. If you really want it to have someone's tomb inside it could belong to whoever found that thing and restored it far enough to use.

there's a series of photos in there that show the process of that's machine's development that shows at least two prototypes made before, one identified as the dkomotive and the okomotive that the protagonist of the game uses.

I would also think twice about number of prototypes. That thing doesn't look viable for warfare. At all. Just look at it's stubby little legs. They are just begging for zebra saboteur to blow the hell up out of them, causing it to fall over at worst moment possible. So I have hard time imagining that someone would build not just one but two prototypes.

Anyway, that all assumes that it was Pre-War design. If you want it to be something else, since I find it hard to believe that anyone in is Wasteland able and willing to build something like that from scratch I would recommend for it to be Raptor refit. Basically someone found crashed Raptor (because there should be at least some) restored it somewhat, but with lack of pegasi magic but abundance of material converted it into walker.
Though in that case it would be easier and cheaper to mount it on tracks instead. Less vulnerable to things like actual Raptor showing up and shooting off it's legs from behind cloud cover or someone toppling it with pit trap.

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so how about the okomotive? the smaller vehicle that's powered by the same technology as the landcruiser?

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Well, this thing gives me Tsar Tank vibes so I'd say that okomotive chasis could be failed tank prototype that was found and refitted by someone after bombs. Basically whatever was supposed to power it replaced with spark-generator or something similar, given some token maintenance and then used for something. Though this design honestly wouldn't be viable in Equestria, both Wartime and Post-War one. It would fail miserably as soon as it leaves roads, especially in rainy and muddy place that Equestrian Wasteland is, and on roads it would need clearing and repairs to get anywhere.
So I'd just mention that yes, it existed, but no, it wasn't usable so it got scrapped/abandoned, mainly because if someone has resources and tools needed to build that thing or even make it viable they probably could restore actual tanks abandoned when bombs hit.

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Nooooo, don't copy the Prydwen. That's a recipe for disaster.