Once you get to the middle of the country, and certainly west of the Minnesota-Iowa-Missouri-Arkansas-Louisiana tier of states, you will find yourself using a lot of two lane road. We are not blessed with the closely gridded network of freeways and superhighways you have in the east.
In the western parts, most of the two-lane US-numbered highways are in better condition than the Interstates, and are often lightly traveled. However, they do go through towns, and you have to slow down.
The few Interstate highways we have carry very heavy truck traffic (both density and weight of the trucks), most of it at higher speeds than you are accustomed to seeing, and the roads bypass almost everything interesting, going from major city to major city (and there may be only one major city per state).
East of the Mississippi, if you want to make good time it is best to stay on the Interstate, no matter how crowded and beat up they are. This is because the towns here are closer together, the speed limits are lower than they are further west, the roads are older and often narrower, and for much of the year you may encounter slow-moving farm equipment on the highways.
I regularly travel between central Kansas or NE Oklahoma to northern Indiana and south central and southeast Michigan. I routinely use the "two lanes" (which are often four lane divided but not controlled access) until I cross the Mississippi, and then take advantage of the uninterrupted travel on the Interstates, unless I have some stops that are not on the Interstate system, and I allow an extra day for that section of the trip.
If you are concerned that Interstate 70 is only two lanes each direction, once you are outside urban areas they are all two lanes each direction.
