I am planning for a cross country ride post pandemic. We have done numerous week long bike trips and a lot along the GAP, which is gravel. I own 4 bikes, only 3 of which I really ride anymore; a 1998, Litespeed Classic (my go fast bike); a Scott Scale front suspension; and a new Surly Disc Trucker. We have done all the long self supported rides in the past few years on my Scott and I can tell, that you for me, the riding position and frame geometry do not suit me for a long day of riding. My butt and back are killing me by 2:00 pm and I move to the 5 and 10 method in the afternoon - 5 miles, 10 minutes off the bike to stretch. When I mountain bike I am usually moving from hanging off the back of the saddle then all the way forward for climbing, and on the pedals over rough ground, but that is not what you are typically doing on level ground or pavement.
I have done numerous multi day, 100 miles a day fully supported rides with the LightSpeed (drop bars) both here and in the Alpes without the same problem. I bought the Disc Trucker and have swapped the bars for the classic European touring bars (aka butterfly bars) with a stem the allows for height adjustment. I have not done any real miles on this yet, but as others have said, this is really personal preference. I think you need to spend several back-to-back riding days, with your bike loaded as it would be on your trek, and see what hurts and focus on that. While you tend to get stronger as the tour gets longer the things that nag you or give you pain usually get worse.
