A large part of the challenge is definitely mental. Staying sharp on the bike, day after day. Putting up with each other is another mental challenge. We all have our off days and can be on the grumpy side. You have to be aware of how each person's day is going and give them space when necessary.
There are two challenges to traveling each day. First... you have to unpack, set up, tear down, and repack. That definitely gets old. You get a system down pretty quickly, but it's still consumes a lot of time each day. Doubly so when it's wet in the morning.
The second, even more challenging part, is coming up with a place to stay each night. That has got to be the most difficult part of traveling "on the fly". It takes up a lot of time... discussing options, trying to explore options on a tiny phone, calling hotels when necessary. I waypointed a ton of campgrounds ahead of time, but you don't always end up near one at the end of the day. Plus, a lot of them are now booked in advance. I think a large component of this is no one wants to make the call on which option is best...then you ride three more hours, get there, and there's a problem of some sort. No one wants to be stuck with the responsibility of having picked a bad place to stay.
The physical aspects of an extended ride are the easiest to overcome. All three of us made a point to get "in shape" before the ride. We all started this a couple of months before leaving. We each chose different training regimes, and none of us had any problems from a stamina point. Sure.... you have the sore butt the first few days, but you power through that.
I will post a screenshot of our intended track when I'm on my computer. I say "intended" because there is the route we intended to ride, and then the route we actually rode. They definitely differ. I didn't bother tracking our actual route, but one of the other guys might have...