Hi GeoffT,
I’ve been fortunate enough to do many long haul trips to the USA, Central & South America, South Africa, Namibia, Zimbabwe, Uganda, plus China, Borneo and Myanmar. You need to accept that the flight will be, of necessity, a long one and then decide how you would like to fill the time. Obviously, in-flight meals will help to pass some of the time and hopefully the in-flight entertainment system will provide a decent source of films. And personally, I always take a few books as I enjoy reading. Try and get some sleep - I used to be hopeless at sleeping on planes but I’ve got a little better at it over time. The cabin lights will be dimmed during the night hours so that helps.
If finances allow, you might want to consider an upgrade to premium economy (if available) which would afford some additional leg room and slightly better meals. I’ve flown Business Class once (only because I got a very good deal on flights to Uganda) and admittedly that was a whole different ball game with great menus, service and seats that convert to flat beds etc.
You don’t say where you are thinking of going but New York (one of the world’s great cities!) would be a great option, Covid permitting - the flight isn’t too long and America’s East coast is only 5hrs behind UK so jet lag shouldn’t be a problem.
Anyway, just reconcile yourself to the fact that the flight will be longer than you’re used to and then plan to fill the time effectively. Look upon it as an adventure and as part of the holiday (easier on the outbound flight, admittedly) and the time will - literally - fly by!
Hope this helps and happy travels when we are allowed to explore the planet once again.
Cheers
Mike