How do you guys concentrate for 4.5 hours?

[Paradox]

This isn't surprising. By the time you've reached the 13th or 14th hole, you've been on the course for about three hours, in the heat, thinking, walking, swinging, and you're not at your mental or physical peak. A letdown at this point in the round is quite understandable. But since you know it's coming, you can prevent it.

great advice right here

[JML22]

my conditioning is great in terms of cardio, soccer 3 times a week and what not but i duno maybe i just dont use the muscles used in a swing often enough.
a aroudn 12th or 13th my drives start slicing as i get out of sync then by the 17th / 18th tee i dont know where the hell they're goin
then my irons stop drawing and i'll end up in a bunker or something then i just care to take the time to setup a good shot at like 8 pm

[clubchamp]

I like to do stretching and keep my fluids up if the round is dragging on too long. If the round starts pushing 5.5 to 6 I will pretty much lose interest.

[JML22]

some rounds just draggggg
i drank 3 bottles of water and ate a ton last round

[Paradox]

some rounds just draggggg

I routinely drink 5 or 6 20oz bottles of water per round..Hydration is a BIG key(plus I'm an "above average" sweater!)

[GaijinGolfer]

What I find helps the most is to think of it as just a game and have fun. If you are so mentally focused all the time you will put so much pressure on yourself that it wont be fun anymore.
Id also agree with those who said that you become solely focused on the game when you are taking your shot and then you relax between shots. If you watch Tiger you will see that when he is setting up for a shot he is very serious and focused on it, but between shots he is laughing and talking with his caddy and the other players in his group.