Particularly during the day it's not that bad - The seats on the 777s out of Heathrow are 9-abreast (so more comfortable than many) and a ~7hr flight during the day isn't a big deal.
+1 on some good headphones, a bottle of water and for me I'd be going an aisle seat in the middle section of seats - potentially toward the rear if the flight isn't full should/could see you with no-one next to you which is a big boost for comfort.
Something to keep you entertained (as above, tablet or phone w/ movies, or a good book) and some music downloaded on your phone.
I'm flying in Y w/ my partner to ORD next month, don't have any issues with it on a flight during the day, for the money. For the return, we're in J because I really dislike overnight flights in Y personally (If I have the time, I'll always prefer to fly during the day and land in the evening) and cause I found a deal that was worth the money for me.
Worst case, if you're on the redeye return grab a good eyemask, earplugs, neck support and some melatonin and try and get a window seat so you can lean against the wall. It's an even quicker flight back (~6hrs), so by the time they've done the service you'll prob have about 2hrs of darkness before the lights are back on for breakfast. Even if you managed to get 4hrs of broken sleep you're still going to feel pretty gross when you land, but that night you'll have the best sleep you've ever had (and make sure you try and stay awake until at least 8 or 9pm else you'll be screwed up for days!
Melatonin also helps your body clock readjust across timezones - it's not a sleeping pill but it will help your body clock align itself again. Can get it over the counter easily in the US.
Above all, enjoy your trip!