I honestly think the hardest part of any novel is the part from 20,000 to 40,000 words.
I've often said that novel-writing is like running a marathon. The latter is something I've never done, of course, but I imagine that running a marathon is a process of pushing through the pain, forcing yourself forward with the knowledge that there IS an end, and while you can't really see it yet, it's real and you can get to it if you just don't stop.
20,000 to 40,000 words is about that.
20,000-30,000 is a swamp, usually. It's just murk and mess to slog your way through. And there's stuff in the water that you can't see, and just maybe, maybe, you're a little lost.
For 30,000-40,000, though, it's out of the swamp and it's just a long, hard trek uphill with a steep incline. Here it's less about confusion or uncertainty-- really once you pass 30K you pretty much are past the point of no return and you will most likely actually finish... and for this it's just work and pushing through the weariness.
Also, I find that once I crack 40K, once I get over that incline, and it's a coast downhill to the conclusion.