The longest I've ever been on a plane is 2 hours and that was a lot, what is it like to be on a 24 hour or longer flight?

I'm 16 and I live in the US, so I've never been on super long flights. I'm gonna be moving to Vietnam in two weeks, so I'm gonna be on a 20ish hour flight to get there, and then that same flight time again 2 years from now to get back. I'm mostly curious as to what a flight like that is like. I typically get bored on a flight in less than 30 minutes, so I can't imagine how boring it must be on a flight lasting an entire day.