The big pitfall of planless writing is that the story will go nowhere.The questing party will wander around the mines of Moria in the dark and never get out. The hobbits forget about the One Ring and become involved in the court politics of Gondor. Aragorn and Arwen hop into bed and suddenly the novel becomes 50 Shades of Grey. Saruman gets involved with flamewars on 4-chan and doesn’t imprison Gandalf in Orthanc.
This is unacceptable. You, the author, have to keep control of the thing so that it goes somewhere. You may not know the plot, but you know when it runs off the rails. It has to end right. You must see to it that your readers close the book and say, ‘Wow!” If they say, “Waitaminute, that’s not an ending,” then you are in trouble. You must and shall shape the thing properly. You are not allowed to cheat your reader — or at least you can’t do it more than once. Remember the end goal? It’s not the Ring in the volcano. It’s a good book.
But now that you’ve written it (or most of it) you can brood over it. Consider the entire work coldly, as if it’s T.S. Eliot’s patient etherized on a table, and pick up the scalpel. Cut out the extraneous stuff (save it in another file somewhere) to tighten it up. Suss out where more stuff is needed, and create those patches or splices. If it’s boring here, why is it boring? Either omit it or fix it. I have written two or three endings for a book, fumbling for the right one. It’s there, I know it’s there, and all I have to do is find it.
Figure out what the theme of the work is, what it’s really about, and start driving everything towards that theme. A good book is almost never just written. It’s rewritten, hammered out on the forge of the word processor. Does it save time, to outline the thing down to the paragraph level so that you don’t have to massively renovate the first draft? Yes it might — if you can write it that way. All the work has to be done, one way or the other. Whether it’s at the front end or the back end is up to you!