Dan, you have nothing to worry about mate.
The key for me is to pick an aisle seat (contrary to what everyone says) near the back of the plane as often where the emergency exit is there are 2 seats instead of 3 and a bit more leg room. Failing that near the back of the plane as there is more space to stand up.
Deffo agree with walking about every hour as it helps relieve boredom and helps your muscles from cramping up.
As said above most long haul flights have tvs where you can watch films (usually a few months old films), tv shows (usually Friends, Frasier, Faulty Towers, Only fools etc) or listen to music. They even have some very basic games to play. I recommend getting some books to read also and stay away from the provided inflight mag as when you do the maths about how many people have touched it, spilt stuff, dribbled on it etc then its more hygenic to leave it well alone
You usually get 2 or 3 meals which can vary from being cold and cardboard tasting to OMG why is that so hot. My advice is to take some additional food with you in your hand luggage. Drinks are fine as you often get as many drinks as you want.
You could always see about getting a very cheap flight from Norwich to Amsterdam etc as thats only an hour and see how you get on. But personally the only thing you'll notice about the plane is the take off and landing, the middle part you could be anywhere.
Am jealous.....Australia is a place I'd love to visit at some point.