>> If you drink a gallon of water you won’t remain in your seat for 3 hours. You’ll be up walking to the loo <<
Not necessarily. The exceedingly dry air in the cabin means you’ll lose a lot more moisture via your skin than you lose in ordinary ground-level conditions.
And by the way, I didn’t mean to suggest that you drink that gallon all at once. Just space it out over your 17-hour flight — and you’ll be fit as a fiddle when you land.
I thought you were recommending having that much in your system before you get on board. But drinking a gallon while on board can still get you up quite a few times during the trip.
Just look at the number of people who are constantly getting up to go to the lav now. Even on flights less than two hours, the lines can get long. Most people in first class go infrequently, they plan ahead because they know how bad the lavs can get especially after ten people in coach decide they’d like to try up front.