Yep, just under 80mph, and it was open road (image in the comments section) so very little slowing down or stopping. Loads of overtaking though, I counted a total of 167 triple road-trains going in either direction. (Triple trailer trucks)
Driving for long periods takes a toll as you've pointed out but in big countries like yours and mine it's required I guess. I think your Montana drive would have been more picturesque than my recent trip though.
Driving through Glacier was great. Then you leave the forested mountains and hit the plains and it's like hitting a brick wall except the exact opposite. It opens up and there is nothing. Just flat land with brown grass. And it's like that until about 2/3 of the way eastward across North Dakota. Then the lakes and deciduous forests start.
Yeah, that sounds a little like where I drove the otter day...flat, featureless ground mainly although it's the Riverland, so Australia's longest river snakes through it which is the photograph above; 2508km long and runs through three States with some very beautiful scenery along the way.