Given that you're in F, I don't think you'll have a problem. I recently did IAD-PEK in C with 2 small children, and it was quite comfortable and didn't feel nearly as long as I expected.

If your 12 hour layover is during daylight hours, I would highly recommend that you consider using the layover for a quick trip to the Great Wall. You can easily book a car to take you from the airport to the wall and back (see the Flyertalk China forum for many discussions on this), and it is well worth seeing. Plus, it will get you out of enclosed spaces for a bit and give you a chance to walk around, before embarking on the next leg of the trip.