I've only been on 2 long trips on my bike. The first on the first incarnation of my bike, clubmans, rearsets, and 1 inch of neoprene as my seat "cushion". About 170 miles each way, about 650 mile total over a 4 day weekend. I could usually go for about an hour before I had to stop and stretch. My hands would go numb and my ass hurt. Getting off the bike for about 10 minutes was enough to get going again. Towards the end of the trip, my ass could only handle about 45 minutes.
My second trip was a couple weeks ago on the second incarnation of my bike. Drag bars, stock pegs and more seat cushion (1 inch of camping pad and 1/2 inch of upholstery foam). This trip was about 320 miles each way and over 800 miles over a 4 day weekend. This was significantly more comfortable, but I could still only go about 90 minutes between breaks. Occasionally my hands would start to go numb if I wasn't careful about my body position and again my ass started to hurt after awhile.
So, higher bars and more seat padding help. Before my next long trip, I am going to make an additional seat pad that goes over my seat.
Like Krafty, I like to stop anyway to see the sites. A motorcycle trip should be about the journey, not the destination. However, since you are in the midwest, I understand there isn't much to see. But sometimes those small towns you pass on the way will surprise you.