Speaking for myself, it's usually at this point in the challenge – when we're just past the half way mark – that things tend to go a little crazy for me. If I have an outline, I will start to wander off track. Maybe a new character waltzes in and decides to take over; maybe my current characters decide that the plot is too boring, so they throw in a couple of explosions; or maybe they get themselves kidnapped by a random villain and then suddenly there are zombies involved and oh crap, what did I just write?
When I don't have an outline, this is about that stage where I start to realise I have NO. IDEA. what happens for the next ~20k, resulting in general panicking and flailing of the arms and more or less getting nothing done. Either way, I've usually started to get somewhat desperate and am wondering why I got myself into this stupid mess in the first place. And I start thinking: “Maybe I should just give up and take up an easier hobby, like stamp collecting. I could do stamp collecting. There are fewer zombies.”
What to do in that situation? Frankly, the only thing you can really do is keep writing. In my experience, the first week and the last week are fairly easy; the former because you start out high on optimism and determination, and the latter because it's nearly over, and once you're past the tipping point (say, 40k) you're so close to the end that the rest of the story just seems to write itself. But it's weeks 2 and 3 that require the most effort. Life gets in the way. The story suddenly doesn't make sense. Your main character ups and dies on you and you're left with nobody to carry the narrative. Whatever happens, it's likely to make stamp collecting sound really, really enticing.
Do not give in to the siren song of stamps! Even if you're in a mess right now, or several thousand words behind, you can still turn this into a success. Stomp on your inner editor and bribe yourself silly with chocolate: whatever it takes to keep going. I was talking to a friend of mine last week about that magical point where, without warning, things begin to make sense, and a throw-away line from chapter 1 suddenly reveals itself as a clue to the ending, or a character you saw as comic relief develops a heretofore unexpected significance. I'm sure you know what I'm talking about. It's that shining moment when, wonder of wonders, the story becomes clear to you and all that slogging in the dark and the grasping for just one more thousand words begins to seem worthwhile. That moment will come – but only if you keep writing.
So. That's my motto for this week: hang in there. Remember, no matter how weird or wrong or stupid it seems at the time, anything you write over the next few days could prove extremely important or useful later on.
Except the zombies. They should probably go.