That was one hideous, but helpful and polite elevator operator. I believe he's only referring to this megastructure being 27km deep (at its deepest) since this is the one he's connected to. The megastructure are the thick plates between The City's floor levels, so there's levels below this one. Each level is gargantuan in its own right. The next megastructure could be hundreds of kilometers above them for all we know. And maybe the elevator can move forward and backward as well instead of just up and down judging by what ugly said, which would add more distance to their trip. Crazy how they have to travel 780km in a goddamn elevator. At least Killy was able to turn himself off, but how did he come back on? It also makes sense that it would take 800 hours or 4.7 weeks to reach their destination. Its a long time, but they're travelling approximately 0.98 km per hour and that sounds about right for the speed of your average elevator. Assuming the speeds are basically the same in this fictional world and ours for elevators.