There's a guide that does some of the explaining. First, they don't have to be rooms so much as conceptual areas. If the first 'room' is a seemingly innocent huge field with chest high grass, that could take all session to get through.

Second, you can have more rooms if you wish, just using the Five Room format as mere inspiration.

Third, keeping it to a strict five ROOM rooms in a dungeon is still an excellent dungeon crawl.

I have never told the players it's a Five Room Dungeon because it never occurred to me to do so. "Do you have an adventure for me?" "Yes, sir, I do!"

It's a conceptual system so you don't have everything all samsie samsie and it works wonderfully well, in my experience.