Yes, the 1000 miles races are about 4 hours long for each race (unless you use something ridiculously quick like the Buick Special or Shelby Cobra, but then they're not really "races"
per se) and the only way to have a break during each race is to use the pause function.
I suggest pressing "Start" to pause the game, then using the down arrow to scroll down and highlight "Screen Settings", this way if anyone was to accidentally press "X" or "O" on your controller, it'll go into the screen settings menu instead of unpausing the game.
Don't worry about the mythical overheating issues if you're leaving the PS2 on, I've been through 7 x 24 hour races and the PS2 has never hiccupped despite being left on for a total of 523 hours in one instance!
(I'm currently 1 hour into the Fuji 1000km race for the 3rd time and my PS2 has been paused overnight.)
Famine has also done well over 400 hours in one instance on a newer Slimline PSTwo which dispells the myth that the newer machines are prone to overheating. (Do make sure you've got adequate ventilation though!)
That said, the 1000 Miles! Event is great fun and I'd highly recommend that you take the time to drive it from start to finish in something relatively competetive. (May take a little digging for a good AI lineup though.
)
As
mafia_boy has mentioned, you can save the game in between races, just like you can with any championship event in GT4.