I usually have an ending in mind when I start a book. The fantasy series I'm working on with my good friend, A.L. Brown, is actually a good example. Although our "end" turned out to be the "big end" of the series, rather than the actual end of the book, we had something to work logically toward. We had something to anticipate.
This, however, has not been the case with my mystery novel. I deliberately avoided planning the end. I planned the crime, had a rough idea about who the villains were, and planned several major plot points. But, I wanted a mystery and so I wrote as though I was going through it day by day, with my characters. It's been a long, but mostly fun process.
And now, within chapters of the end, its become a slow slog toward the finish line--at least as far as plotting goes. I don't know exactly how this thing's going down. It may get messy. Anticipation is weighing me down. I want to know how it ends and the tension is crippling...
Maybe that means it's good, eh?