Having some food and drink relieves the boredom of long flights.
Having somebody to talk to is also very good.
I've done up to 6.5 hours to date in one go and it was OK. Much depends on the scenery (both down below and in the other seat
). Flying back home over northern France is never going to be exciting...
Oxygen helps to keep one awake in a way few of the "hairy chest" types would believe. If I am going home, and I have enough left, I turn it up and use it up. But then I never fly airways below FL100, which is a bit marginal without o2 for long legs.