Welcome!
The Keys to the Kingdom tour will take you into some of the Utilidor, mostly the area up at the front of Main Street (the cleanest area of the tunnel!). Since you can't see all of the Utilidor, I'll describe some of it to you--well, I'll describe how they look since that won't get me into trouble.

Most of the tunnels are dark and damp, with condensation (commonly referred to as "tunnel juice") dripping off of pipes. Every so often, when trash is emptied into certain bins, a vacuum system sucks the garbage to one location, and you can hear it rattling down another system of pipes. In each hallway, the bottom half of the wall is painted to correspond to the land above. MS' tunnels are burgandy, AL's are green, Fr.L's are orange, LS' are blue, FL's are pink, and TL's are purple. Although TT doesn't have a tunnel system, some of the entertainment area is considered to be TT and has red walls. The tunnel system is essentially a big circle around the park, with one long, rust-colored hallway shooting straight from FL to MS. People walk all over the place and little Pargo carts speed through the tunnels, beeping every time they turn a corner. --whew!--
Although the tour only shows you a smidgen of the vast tunnels (you see the area under MS and the computers), it's the only way you'll see the Utilidors without working for Disney.
Tangent: Epcot has a small tunnel system in Future World. Unlike the MK's, it's spacious, clean, and (you guessed it) pink concrete.