The 250 Ninja has been around for over 25 years and believe me, this is a well made machine and can handle just about anything you can throw at it. (you will get tired before the bike will) The 250 Ninja is by far one of the most versatile bikes around. Look around at some of the other 250 forums and the number of riders still riding their older model 250's all over the place, the iron butt riders (long distance/endurance), track racers, land speed racers, tourers even some off road. The 250 Ninja does it all, unlike some Supersports. If this is your first bike, it is the perfect thing for you to learn how to ride.
Like I said, you can do it all on a 250 and be absolutely fine with a 6 hour trip. Just make sure you do the necessary maintenance and preparation before your trip (keep in mind that hwy riding at high speed might require a sprocket to reduce engine rpm). Honing your riding skills is really important and the 250 is hands down the best motorcycle for beginners to professionals alike. Until recently, it's been in a class all by itself for a longtime, so it's longevity is a testament to it's reliability and owner loyalty.