Just starting this so I'll remember to come back and add my thoughts after I hear the second half...
Quick first thoughts, session zero is way too soon to be 'writing the ending' or even just setting the stage for a couple of reasons. First, given how rarely campaigns make the long run, it just seems like tempting fate. But more importantly, I find that when I "know" the ending, I tend to be more linear in my storytelling and less responsive to the characters as they develop. I do agree that you should start thing about the ending well before you actually get there, but if you're gonna run a multi-year campaign, I think you can wait until the last year or so...
More to come...