Is King's 11/22/63 Ending Given Too Early?

I have been reading Stephen King's latest novel release, titled 11/22/63. I have covered 80 pages and believe that, so far, it's King's best novel since his incredible Lisey's Story from 2006. However, I need to interject a concern. I hope I'm wrong.

It is no secret that 11/22/63 is about an individual in "our" time stumbling across a doorway into the past. He uses this time portal to set about preventing the assassination of President John F. Kennedy on the featured date. Fine, no problem from me with that, except that of course it's been done before in other stories (and about other assassinations). But King makes this version very different (so far) from those other stories with the manner in which this time passage takes place. Still no complaint from me. So what's my problem, anyway?

It might not be a problem, but rather a deliberate ploy by Stephen King, a kind of literary misdirection to lead the reader to erroneous assumptions about the plot flow that will later cause astonishment when they are exploded later in the story. However, I think nevertheless that, after reading only 80 pages of this 850-page novel, I believe I already know how it's going to end.

All I can say, without giving away too much of the story, is that it has to do with the ground rules for time travel between the past and present that King lays out. And that last rule, appearing close to page 80, screamed "preplanned ending" to me. But then again, Stephen King has maintained that, when he sets out to write a story, he himself usually doesn't know how it will end. He develops the characters, puts them in usually distressing circumstances, and then lets the story unravel to him, as if another party were telling it instead of his own imagination working overtime. I can't argue with his explanation for his writing technique: he is easily my favorite author. But with 11/22/63, did he stray from this and plan out the story's ending in advance?

Hopefully, I'll be reporting at some time in the near future that my concerns were groundless. We'll see...