Well, there ought to be horses on a Pony Express I think
( sorry, couldn't resist ! ) I assumed you were talking about an overland route so I had to look it up - that looks a fantastic drive, but I don't think I'm up for it yet, still a novice off road driver. In fact, at Overland Expo PNW this year I signed up for an offroad driving course, which was eye-opening, sphincter-clenching, and amazing
I had a professional driving instructor sitting next to me and they took me up and down inclines, around off-camber turns, over log obstacles, through deep ruts and pits and over rocks and bumps that I would have never dared to try ! On one of the turns with a noticeable off-camber element to it, I was gravitating towards the "safer" side of the turn and the instructor said "keep over this way", and I was like "noooooo!" At another point he had me stop on top of a steep incline and asked me which way did I think my steering wheel was pointed - so I clicked on the off road pages on the truck and said "eh,looks like 8 degrees". And he goes " turn that thing off ! What are you going to do when that breaks?"
Anyways, it was a great course, and I already put what I learned into practice on the recent trip in SE Oregon. I drove over some stuff that I would probably have balked at previously.
But I think I need more experience and a buddy in another vehicle before tackling something like Pony Express.