There is really no way to accurately model the distance between towns on a model railroad unless you own a large stadium or aircraft hangar. A six-foot run in N scale is equal to 960 feet--how many towns do you know of that can be completely represented in 960 feet? That's 320 yards--maybe two city blocks.
The way to do it is to cheat. Things like view-dividing backdrops, scenery tricks, forced perspective, shadow boxes, limited perspective, and other ways to make the viewer think they're seeing more than they are.
Simplest, though, is the backdrop. Say you've got a 4x8 loop layout. From just about anywhere on the layout, you can see the whole loop. Kinda boring. But by sticking a backdrop in the middle of the layout, something maybe 2 feet high and 6 feet long, you now have two 2x8 foot areas totally invisible to each other. So you can model a bustling city yard on one side, and an industry out in the middle of nowhere on the other.
Your U-shaped plan is ideal for this--just design three scenes, one for each section, and the corners are transition areas between scenes. If you're standing in the middle of the U, your attention is focused in one direction--you have to turn 90 degrees for the other sections to be visible. Which allows that division of space to take place in your mind instead of on the layout. If you have access to both sides of that large space, you could even put a divider in the middle of it and have, potentially, a fourth visually separated scene that wouldn't be visible at all from the rest of the layout.