In the west you will find two-lane speed limits of 65 or even 70, and faster in Wyoming and Montana. Towns are far apart and 400 or 500 mile days are fairly easy. In the Rockies things slow down a bit but not at all like Appalachia. In the east the big hazard is congestion. In the west it is: thunderstorms, heat, wind, and running out of gas.
Plan on a good method to stay hydrated, wear good gear, gas early and often, and have fun.