Arguably, writers actually spend relatively little time writing. There are a lot of other things to be done in the process of building a story. Characters need naming, places need researching. Fights need choreographing. That one sentence in chapter three probably needs rewriting.
A large amount of time is spent on editing and rewriting. When facing a mountain of words to go through it can feel a little daunting, and finding somewhere to start can be difficult. Here’s a few places to try starting.
Read through your manuscript and make notes as you go. This can help you see what absolutely needs fixing at this point, and what can be saved for a later draft.
Outline your plot again. Try doing this from memory, and then check it against your MS to see where you might have extraneous scenes, events, or where you might need to flesh out some scenes again.
Break it down into multiple drafts and pick a focus for each one. Perhaps in one draft you’re focusing on getting all the character backstories in, and the details. In the next you might worry about plot structure. In the final draft, go back and clean up your voice, word choice and yes, grammar.