It's all in the staging
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Consider the usual way these big, fast projects are done: "10 miles of track laid in one day", that skyscraper, freakishly fast railroad bridge replacements (wait til morning Amtrak, replace 800' bridge, let evening Amtrak through). These absolutely depend on advance placement of those materials, and lots of advance site prep. These quick builds are generally stunts, and everything they'll need from modules to tools is already on the site.
If you can't do advance site prep, it'll help to at least reconnoiter the site so you're working from a really good site map. That said, Seabees can do site prep pretty fast. Trouble is, this is linear: first you must reach the site with the prep equipment, then prep, then start to build. Tick tick tock.
Fly sections in requires cutting-edge tech
You need either favorable wind, or helicopters that are really good at automated dynamic positioning. *Another answer proposed delivering the modules to the vicinity and also having a crane. Problems: setting up the crane is still a critical path; helicopters don't play nice with cranes; a safe distance will mean a big messy laydown area.
So the answer is the helicopter is the crane. Build the modules out of ultralight materials - spare no expense. That way you can haul more building per pass. Then you have embedded radio-tech (i.e. Bluetooth) in the modules that talks to the heli-crane about positioning. The automation is to keep the humans out of it, because when humans are involved, Desert One happens.
Upside: nothing touches the ground except the building and site-prep team. Their gear can be landed inside the building's footprint, and a space in the first floor can be dropped over the construction gear.
If you're able to do all this, you may also be able to revive the old super-helicopter designs like the Mil Mi-12.
Each module dangles 4 cables (or 2 U-shapes). As the module gets near, workers in the module below (which has an open ceiling) grab each cable and feed it into a pre-installed winch. The winches, talking to each other and the positioning tech, drag the new module down, helicopter and all. The helicopter's up-lift keep cables taut so the module comes down square. Once the module is locked in, the helicopter slacks its cables, drops load, and goes to get the next module.
Keep in mind, this is not what engineers do. They think carefully, and take the time needed to be safe. Speed has to be engineered into the design and the robotics.
Haul Road to Hell
You want to go overland? OK. You have a logistics train to get all the materiél to the site. This sounds easy until you mention no roads.
Here's the gotcha with that. If the area is at all civilized and there are no roads between towns 20 miles apart... there'll be a reason for that like a wide river, deep canyon, or truck-eating swamp in the way. Obviously, terrain has everything to do with how you solve this.
So you get your Seabees out there and build the road. The combat engineering isn't necessarily a problem... but this is all happening inside your 20-mile limit, so does it happen "on the clock"? Combat engineers can move pretty fast, but that really depends on terrain and you could find yourself unable to build the road inside 12 hours let alone the building.
There are two ways this can go sideways. First, you can hit complications building the road, and have the entire project run down the clock because it can't get to the site to start. It might be worth having 2-3 separate Seabee batallions working redundantly on parallel roads, so you can zig-zag if needed.
Second, you can have a similar, critical problem on the onsite construction.
All of these delays stack. Like how the Empire Builder is always hours late. A 56-hour train run just faces too many chances for a grade crossing accident, traffic delay etc. French TGV runs are on time because their runs are short.
Erection is straightforward, just have the usual jacking crane in the middle of the building and abandon it rather than replace it with elevator shafts.
Alternately, jack the building instead of the crane, using Hulcher bulldozers or built-in jacks to lift the entire extant structure while new floors get slid into the bottom. Let down onto them, latch them in, then jack them for the next floor. Faster and less messy.
Burners come closest
Every year, Burning Man attendees roll in with their crazy structures and plop them down on the virgin Blackrock desert. They are prefabbed (forget about getting any time on machines at TechShop in the month prior), modularized, put on vehicles, dragged in, and set up.
2013. The saucer is 120 feet across.
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How long? Well for Burning Man official structures (you know, like the Man himself), a week or so. But for some attendees it is pretty much overnight, and if the bigger projects were pressed, they too could get it done in 12 hours.
This is done by geek kids, for fun. Imagine what an advanced military could do.
Fly the building there.
This assumes your building's purpose can be accomplished in a reasonably large airplane or starship. It can look like a building; nobody needs to know the first few floors are nothing but elevator lobby and engines.You build the building at leisure. You only need to land once; consider The 100, where certain former spaceships landed roughly, but continued in service as a building/fortress.
Or just drive it!
Perhaps stretching your meaning... but we build the building, at our leisure, back at the staging area. The first 3 floors are this.
source: NASA
On Construction Day, the building is already finished. We start driving whilst the combat engineers sprint ahead of us, blazing the road. At 2.0 MPH, we get there with 2 hours to spare. Since there's only one vehicle passage, there's more time for the combat engineers to tidy up/coverup the obvious haul road, and they don't have anything else to do. The huge tracks can be hidden by fold-down covers.
Obviously this doesn't work with a canyon or swamp, but for a wide flat river like the Platte with good rock beneath it, it might be just the ticket.
