Depends where in the US. You're unlikely to set your cruise control and leave it in a big city, unless it's late night/early morning.
Outside of the big cities, it can be much more relaxing. I drove across the mid-west and did the same as you. Drive at 70-80 mph and not take the cruise control off for hours.
Sure, as an Austrian I prefer driving on German autobahns as well (due to better perceived discipline), but the US trip was something else entirely - and I've come to think that it's maybe because almost every car has cruise control.
It's not even the speed differences between different lanes, but staying in lane, setting CC and not having to correct it for a really long time is not something I was used to, and going 600km or more a day left me far less tired than over here.
EDIT: Maybe there are intra-US differences as well? Since most here seem to talk about California...