One other option that hasn't been mentioned yet is to take a flight with a connection instead of a transcon. There are two advantages to this. First, you get to get off and walk around halfway through your flight. Moderate walking is even better than keeping your feet up. Second, your chances for an upgrade are better on mid-cons than transcons.

If this doesn't work for you, I would suggest looking at mileage upgrades (buy the miles if you have to). You don't want to start out a trip like this with a health problem.