There are certain things that are a given:
1) Day turns into night.
2) I like chocolate. And cookies. And cake.
3) First drafts can suck!
I know suck may be a harsh word, but they call them first drafts for a reason. They have a certain amount of suckage built into them.
The pacing may be off. You might introduce a secondary character only to forget about them halfway through. Oops. Hasty addition time! Been calling your love interest Jensen for 20k, but then discover your MC making out with some dude called Jake? Hmmm. Adverbs bleed from the page. Purple prose That brilliant plot twist doesn't quite fit. Darn it. Who says it needs to be logical? *giggles*
What to do?
Write on! Embrace that suck. Get the words out.
There's a reason books get polished in revision. You can't revise an empty page, but you can revise through the roadblocks and errors to uncover the shiny prose beneath.