I certainly don't sit down and plan a book out before I write it. There's a phrase I use called "The Valley Full of Clouds." Writing a novel is as if you are going off on a journey across a valley. The valley is full of mist, but you can see the top of a tree here and the top of another tree over there. And with any luck you can see the other side of the valley. But you cannot see down into the mist. Nevertheless, you head for the first tree.
Now, seeing the other side of the valley has its advantages. You don't have to ramble around, this way and that, grabbing things at random and tossing them in, in hopes that something interesting will turn up that might, somehow, lead somewhere like a resolution, and then go clean up the traces, which, as J.R.R. Tolkien proved in The Lord of the Rings (and still more, in my opinion, in The Hobbit) can be quite difficult, especially when they have wormed their way into the structure so somethings can only be removed in half, and the other half carefully nursed into a different place in the plot. You don't have wonder what the conflict will be -- it will be the forces that collide at the other side.
But not seeing the other side of the valley has its advantages. Especially if you want part of the characters' problems to be that they are perplexed and do not know which way to go. So important for producing anagnorisis (discovery), and its consequent peripeteia (reversal of fortune) -- things the virtues of which were clear so early in the beginning of literary criticism that the terms are still Greek.
I see this the worst in SF short stories about unraveling a mystery. Frequently, the characters pile hypotheses on hypotheses, never testing, never coming up with alternatives, until they reached the author's explanation for the marvel. The author knew what the solution was and did not sufficiently project himself back into the necessary ignorance of the characters. But part of that is that the discovery is so crucial to the story. In other tales, where it is not central, it can still be damaging. If my memory of The Return of the Shadow is right, Tolkien sent the Fellowship up in the mountains to try the pass before he had them fail, and turn to Moria. It's hard to remember that if you have decided the successful route, that your characters probably ought to at least argue about the point, and perhaps even try a different route, only to fail, before they take it. Adds conflict. Because while knowing what collides at the end is only half of it. You have to make them convincing forces, and knowing which one won, as you often do, you have to resist and make it clear that the ending is in doubt. You can't use the first thinig that comes to mind, the victory of the one you know will prove stronger in the end.
Sometimes, of course, you have to overturn your assumption that you did see to the other side of the valley, because a bright idea turns everything upside down. But it can be a problem short of that.