Middles. Otherwise known as The Hard Part.
Well, unless, for you, beginnings or endings are the hard part. For me, it's middles. (A work of fiction is composed of three parts--a beginning, an ending, and a muddle. And so forth.)
This is where the fine art of recomplication comes in. If the opening of a novel is devoted to setting up the situation, demonstrating the conflict, introducing the characters and the world, the middle is about building and maintaining stakes. (and getting to the climax, of course.)
I like to say that I just keep breaking things until I can't figure a way out, which is pretty accurate. But there's a balance to it--the narrative has to reward (pay off) as it recomplicates, or it becomes boring. In other words, you can't stretch one question for 500 pages. Well, okay, you can. But along the way there need to be mini-arcs, small conflicts and small resolutions, little victories to go with the losses. Otherwise, the book becomes, well, one-note. And can be enjoyed as well by skipping to the end as by reading the entire narrative.
That's all very vague and not very practical, isn't it?
Okay, here. So you've gotten to the end of the setup, you've hit the point where you're not sure what happens next, and you're stuck at the previously discussed Inevitable 30K (35K, 40K) Wall. You are staring the Dreaded Middle Of The Book in the eye, and it's not looking down.
What do you do?
This is the time for the first major reversal. Give the characters something. Take something else away.
Jean Valjean agrees to rescue Fantine's daughter, but he's also confronted with his Wicked Past. Conflicting obligations! And a cop is on his trail! Raise the stakes!
Send in that man with the gun. Kill somebody. Get somebody laid. Hand him the key to the puzzle and then snatch it away. Change it up!
Open three books you like to page 150 and see what's going on. Flip a few dozen pages to either side. I betcha, in most of them, something Big will have just changed.
And then you follow the implications of that reversal to the next reversal, and so on, until everything goes boom.