I was concerned the first time I went to Asia in coach - 26 hours of flying - longest leg I've done so far was 15 hours. I found that once you mentally prepare for it, it really isn't a big deal. You don't say what airline you're flying, but I'm hoping for your sake they have in-seat IFE in coach. I find that playing the video games (and getting addicted to winning one) can blow a bunch of time and get you to the point of exhaustion and ready to sleep.

Plus, it seems that most of the international flights I've been on have better seat pitch in coach than your normal 2 hour domestic runs. A couple inches can make a pretty big difference. I'm 6'0".