Here's a secret to all future DM's, whatever you planned, all those rooms, no matter where the players go, they can still end up in those rooms. What people haven't seen, they don't know it was planned for them, so as soon as they hit a dungeon, you can make the dungeon what you planned before. Give them the illusion of going to a new place and you're making it up as you go along, but it's what you planned all along because it doesn't matter if they enter a dungeon in City A, City B, or Forest A, they don't know what the dungeon was meant to be until they see it.
That sewer? Oh, congrats, you found the secret dungeon, now you're happy and I use whatever dumb shit I planned anyway.