I totally agree with Glen and Poppy Dave, and here's my own additional tips.
Full check of bike fluids and tyre pressures before you go
Fill with fuel before you go, and plan your fuel stops
Wear/Take whatever you may need to be physically warm and confortable
Consider wearing a scarf or one of those "Buff" tubes about your neck
It's easier to take an extra jumper and your winter gloves in the pannier, than wish you hadn't left them home
Wear earplugs, as it's far less tiring to ride when you are not bothered by wind noise
Carry waterproof and windproof clothing as the weather may not be what you expect, or you may need them to keep you warm
Wear suncream on your face, 'cause your nose will get burnt through the visor
Drink plenty of water, and avoid alcohol until you get home
Avoid a heavy meal at lunchtime, which may help to make you sleepy in the afternoon
I don't ride with music, but I know that many enjoy it while riding
Savour the time on the open road, and appreciate what this vast and beautiful country has to offer, as wide open spaces are non-existant in many parts of the world.
Enjoy the ride, you lucky bugger, and tell us how you got on when you get home, 'cause I never want to forget the adventure of my first long rides.