Left foot swollen since a 5-hour flight 2 days ago. Doesn't hurt to walk, swelling does down at night, but returns in the day. Should I be worried?

Concerned. I would be concerned. Mainly because of your age. Usually, the concern on long flights is someone can possibly get a clot in their leg. The concern is that normally your foot is not swollen. It is possible you have a stress fracture, though, i wold expect some degree of pain. A visit to the doc, i think would be a good idea.
Might want to check . This may be just venous insufficiency or varicose vein and such. But it wouldn't hurt to have a doctor take a look to be sure. Also try wearing support hose at least knee high as well.
Rigors of Flight. This falls into the realm of transient intermittent foot swelling caused by you 5 hour flight with inactivity. If this was the first time that your foot became swollen, then the chances are that with a little time and activity the foot should go back to normal. As long as the swelling goes down at night, it is probably only a matter of time before all is normal. Keep a keen eye open though.