You've got that right. Some of the attitudes here remind me of the folks who rode out in their fine carriages to observe the First Battle of Bull Run (First Manassas).
They expected the conflict to be entertaining and amusing. But it wasn't.
I think that most people who say “when the fun starts” mean it sarcastically.
I’ve used that expression, as well as “when it drops in the pot”, “when the hits the fan”, “the balloon goes up”, and others.
When I say “the fun starts” I certainly don’t mean clowns and lollipops, I mean it with all the sarcasm I can muster. It helps me cope with the sense of dread.