For me, I know the story from A to K and nothing else. I get to K and L and M pop into my head. The story is over when it is done even if I didn't reach Z. :)
That's common, but it's usually best if you write towards the ending. Thus you'd want to know where the story is going to end up, so you can help guide the reader towards the destination (which typically features identifying the central conflict, and the various steps the charater's take in resolving that conflict). Even with that knowledge, authors are always surprised by the twists and turns their stories take on their own (i.e without their planning such actions). But that's part of the fun. If you already knew everything that happens in a given story, WHY would you chose to write it? It's like rewriting newspaper articles, there's simply no surprise or excitement in repeating details, unless you can put a different spin on them.
But for REP's final point, very often you'll have the central conflict, but you'll choose to end the story before that conflict is complete resolved. So you end up with a 'partial' win, or you'll leave the characters at a certain point, unsure what they'll do in the future, simply because those issues are outside the scope of the particular story.
Many times I'll write a one-off story, where a character manages to achieve something significant, but I won't write any more, even though the character still has a hell of a lot to accomplish still. But then, months or even years after writing the story, I'll suddenly see a new way to continue the story, by portraying entirely new conflicts rather than simply 'continuing' the old story. That's why I'll write the sequels, not because the sequels are expected, but because the new story is an entirely NEW story in itself, despite it's being a continuation of the previous stories.
But, I'm sure REP will see it a different way entirely, since I've just shoved so many of MY opinions in his mouth! 'D