I did the 3 Day Novel this past September. The word count is half what I get from doing NaNoWriMo but it’s a complete story, though a short one. I went into this with a detailed outline which helped a lot. In the process of doing the writing I hit most of the outline targets but I know that the story is short; some scenes are missing, possibly some secondary stories are not formulated yet, and some elements that I hoped to bring out need padding and reinforcement.
I reread the story two days ago for the first time since the Tuesday after the competition closed but in the meantime I have spent time thinking about where the words are missing. Yesterday I had the image of building a fence and I think this process makes a useful analogy. I imagine a stone fence, like in the English countryside, running over damp green rolling hills. 
The outline is like the first placement of stones to mark targets. The targets are selected for even spacing but connected in a line, row, or however it is imagined that the completed fence should travel. Then with your first writing (rushed, in the case of a 3 Day Novel or NaNoWriMo project) you try to hit the targets and to build them up and fill them out.
Having done that writing, my job now is to re-evaluate the posts of the fence, to see if they connect with each other properly, and more importantly to see where it’s misdesigned, where there are still holes where the sheep or goats might wander through. Maybe I need a whole new secondary fence, one that runs parallel with the original, with posts that fill the gaps of the original. Maybe I zigged left where I should have zigged right, or maybe there’s a huge gap that I didn’t notice where I need to add four more post/targets, or spots where two posts are much too close together and I need to space them out better.