I only had my first trip my plane at 36 years old and have made several trips since then but nothing longer than 4 hours and all on budget airlines.
I would recommend to always checking the airport departure board rather than on your phone as we nearly missed a flight when the website did not update.
For me the first big issue on my first flights was getting used to the plane tipping and turning, you just have to relax and have some faith in the pilots even when you feel that you are pretty close to the ground.
Then on the descent my ears popping on the first 2-3 trips could be painful and affect hearing for a while. I found having a mint or foxes glacier type sweet to suck on helped as did having my noice cancelling ear plug type headphones .
As others have said the noise cancelling headphones help with blotting out other passengers who can be noisy although the worst people have been people with tablets watching films with no headphones. Looking out of the window can be a great way to pass the time if there is a view, flying over the alps, over Denmark and along the Croatian coast it id just great watching the world go by. But long stretches with cloud or just water are a bit dull! For entertainment pre downloaded videos are great and I always enjoy a good puzzle game to loose myself into (the Einstein puzzles can eat up a big chunk of time)
I cannot comment on walking about, using the loo or plane food as I have not needed them. Staying hydrated is important and the air con plane air can be quite dry so we make sure of having plenty of water. Usually we have a pastry or sandwich with us so we can eat when we want.