From New Mexico to Georgia it is flatter than a pancake. It is a piece of cake.Go south of the Appalachians in Georgia to hit the Atlantic and you only have two dinky ranges to cross to get from coast to coast. . After doing really hilly terrain, Can't see why one would flinch. Southern Rockies flatten out compared to a Colorado crossing. How long does it take to do 120 miles across the plains.
I would predict 6 hours once you get into the swing of it.Gives you much of the day to relax and do other things.
I would not be feeling angst, I would be reved up to just do it.