Lose the back pack if at all possible. Wearing one around town or for shorter rides isn't a big deal but if you're planning on doing real miles your back will not be pleased if it has to carry the weight the whole time. If you must have the extra bag, strap it to your pillion seat.
Essentials: water (1-2 20oz bottles just for emergencies), high energy snack food (candy bar, trail mix, etc.), basic tools for things like flat tires and broken clutch cables, camera, battery charger for phone, paper maps of the areas you will be going through, SOCKS (can't even begin to tell you how miserable it is to ride in wet socks once the rain stops and the heat rises again).
Try to limit your changes of clothing as much as possible to save space, you can always launder stuff along the way or when you get there.
Gas: Bring some if it makes you feel safe but it's unlikely you will need it. Rather than relying on a reserve can, be smart and stop for gas when it's available. Don't decide that since you're only 100 miles into your 160-180 mile range that you can push to the next station because that one will be closed and then you're screwed. Top off, take a break and check out the scenery for a few minutes. You will avoid running out of fuel and you'll stay more relaxed and fresh throughout the ride.
Lodging: Book your destination ahead of time. Unless you have a strict schedule for the ride don't book your stops. You have no idea how long you're going to go each day which means you have no idea where you're going to stop each night. Booking everything early means you are likely to either have to push farther when you're tired (dangerous) or stop early when you're making good time (frustrating). Enjoy the ride, stop whenever you see something cool or need a break, and grab a room wherever when you're done for the day.
Fatigue: You are tired BEFORE you realize you are tired. Droopy eyes and such are a sign that it's already too late. Be very very aware of your mental state at all times. If you notice that you're having difficulty holding on to a specific thought, or if you aren't fully processing the things you see, it's time to get the hell off the bike. When the distance between stops starts to dramatically drop because you're butt or arms or neck are tired, it's time to stop. When you start misjudging corners or distances, it's time to stop. These are all clues that you're tired. Pushing past this is dangerous for you and everyone around you.
FWIW I'm not a seasoned long distance guy. I do a 700-800 mile 3-day ride once a year and beyond that I normally stick to a couple hundred a day at most. However, the first and only (so far) really long distance ride I've done was CT to western MT. 3200 miles in 9 days on a Honda 919 going all numbered state roads (no highways or interstates). I guessed at a lot going into that trip and learned a lot along the way. My suggestions above are based on mistakes that I made and a very few things that I managed to do right the first time.