Lots of good advice here, already.
Yes, you need to gradually work up to 400 mile trips, in my opinion.
After riding motorcycles since 1967, I can tell you a comfortable seat keeps long trips from being flatass miserable. Pun: intended.
Prefer Mustang seats, myself.
The ones with backrests. You can thank me, later.
And a windshield adds about 150 miles per day, to how many miles I'm comfortable with riding.
Will I ever own another bike without one? Nah.
Next area to address is foot position/ secondary foot placement. Important to have highway pegs, or some alternate place to rest your feet.
Again: just my opinion.