If you do end up looking into new bikes, make sure that the Ninja 650R is on your radar. A lot of these real sport bikes (600s and 1000s) have ergonomics that are not ideal for long trips. You’re leaned over all the way, all the time. On the Ninja 650 (which is what I ride), you’re more upright, but you want easily lean over to escape the wind or just for a change. It’s also heavier than the 250 and more powerful, so highways are easier. 70 mph in 6th gear is around 5000 rpm.
Disclaimer: I’ve never ridden the Ninja 250, so I don’t really know how they compare.
Also, I’ve heard that pair of bike shorts (bicycling shorts) with the padding can do wonders for long trips. I have a pair, but I haven’t thought to use them.
I second what someone said about carrying as little at possible on my body. I hate bringing a backpack. I have a tank bag (highly recommended), and I’ll get a tail bag before I take any long trips, but I hope to avoid a backpack. I’ll get saddlebags before I wear a backpack.