Hard to say if YOU will be uncomfortable after 400 miles but a good chance not. I've done several 1000+ days on the X without any issue at all. Guess the most on the X in a short time was the SS2000 listed below, 2152 mi in 2 days or about 40 hrs. Done a couple 10K trips too, one to Alaska and a lower 43 state tour. My first 1200 mile single day was when it was new stock from southern florida to northern Pa. My back and neck were very painful, butt was numb and hurting at the same time. Second time I did it back and forth with an ultimate seat and moderate pullbacks Phat II. Still a little shoulder blade pain but not much. Then went to the Phat IIIX risers, I think 6.5" pullback, perfect! Since then with the bike setup I made that trip a few times back and forth to my daughter's always in a single day with no problem.
You have the seat so that is a big help, common to get pain in the upper back and neck from reaching too far forward. You can reduce this with risers that put the handles closer to you and I find it helps to tilt them down a little for a more neutral wrist position. It's possible there may be pullback risers on yours already. Not much aftermarket out there anymore so if risers are not an option a backrest will take the strain from holding yourself forward, but if the bars are in the right position you won't need a backrest, if a backrest isn't an option you can pile your gear on the backseat at the right postion to support your back and cinch it tight with straps, not bungee cords. The floorboards are excellent on the R,S,T but good to have other options for a temporary time to get you out of that same leg knee position, a rollbar with pegs gives you a couple extra feet options. A throttle lock is great for giving the right wrist a rest it also allows you to lean a little different on your backrest just holding with the left hand.
Often overlooked is loud pipes, loud pipes are VERY physically draining. There is a reason street touring and adventure touring bikes are quiet. If you have loud pipes put something in your ears.
I had my bike set up where I said miles are simply irrelevant, more a matter of staying awake for the long miles. My son is also a long distance rider and when he rode my X the first time he asked if I ever fell asleep riding it, said it was like riding a lounge chair on wheels, truth is I have dozed a couple times and it is a terrible experience. In a car your head falls forward, on a bike your body falls to the side and its a rude awakening when you catch yourself.
Pics below; arms in relaxed position almost at side, the other, on cruise control seat and feet in relax mode.
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