No reason why not, people do that on mountain bikes on trails. Pavement is easier. Lots of people tour with flat handlebars/upright posture, or do charity rides that way as well. I've done 80, 90 km rides on a comfort/cruiser style bike at a decent pace.
You definitely want to get your seat adjusted though. You'll use more muscle(both different muscles in combination, and sheer mass), making it a lot easier. Once you get adjusted of course.
in my completely noob, uneducated opinion, saddle and leg position matter more than bike. A high quality/well matched bike will make things easier, for sure. But without good fit and saddle, it doesn't matter what you ride, you'll hit a wall a lot sooner. But again, I'm a noob.
Be sure and have enough liquid and some food. Most bike/running/tri/outdoor shops will have fast digest gels and energy bar sort of things, meant to be consumed while active.
Or you could classic and eat bananas and pb&j's. Or convenience store snacks.
The 'seat' is called a saddle, and it should be so high that your leg is nearly fully extended when the pedal is in the lower position (6 'o clock).
As far as the rest is concerned: take it as it comes, and see how far you get. If you don't already own any, get yourself some biking gloves (they needn't be expensive), and some padded shorts. I usually wear lycra cycling shorts with regular shorts over them, so I don't look like a **** off the bike