I fly between the UK & US two or three times a year and have done for over fifteen years.
If you prepare poorly (insufficient sleep prep), as well as poor hydration and poor diet before the flight then you will suffer. Otherwise you may be just an unfit fat git.
Being a pro footballer or being sport fit is totally different. Don't believe me then look to the MLS, NHL, NBA or MLB where players fly most weeks, possibly not the miles but they often fly through three different time zones.
The end of the day, it's about preparation and fitness and little to do with time in the air, or length of flight.
I'm far from unfit. I'm a very very light sleeper though. Nothing much anymore can do about that.