Remember that thousands of people survive trips of this length in economy every day.

My top tips:

-- Bring your own headphones with an adapter plug. The plastic ones they hand out will start to hurt your ears in short order.
-- Stash some snacks in your carry on.
-- Book an aisle seat. This makes it possible to get up an wander around frequently.
-- Don't prepare to be miserable. It's all about mindset. If you go in moaning to yourself about how awful it is that you aren't flying in PEY/J/F, you'll spend all your time focusing on how uncomfortable you are. If you go in confident that you've prepared to be as comfortable as possible, you'll be much happier.