1) Some flights plus layovers can last for over 24 hours.
2) You cannot take your own food and drink on the plane.
3) You have to eat what the plane provides or don't eat.
4) All you can do is sit.
Answer on How do boxers with strict diets and workouts schedules deal with overseas travel just before a match?
Normally if a fighter has to travel a long distance they will do so 2 weeks or earlier before the fight so they can train at the location rather than at the last minute or day.
You can take your own food on a plane and you can drink water.
You do not have to eat what the airlines provides and you can order special food before the flight, many people order vegetarian or religious meals.
You do not have to sit, I always get an aisle seat and get up and walk at least once each hour.