Griffith just finished from fighting a war, there has to be some downtime for now. And maybe that's why Griffith fused the planes together, to be able to attack Elfheim or something. Well whatever the reason is, Guts and Co. should be fine for now. Well assuming that something else doesn't attack them while on their way to Elfheim
i don't think miura has any plans to finish the story... he's just going to keep moving forward with no end in sight
I'm instead absolutely sure Miura's got a perfectly clear idea about the ending. If that end is near or not, we can only guess. Also, take into account:
1) Mangas, generally speaking, are written in a way which almost always take into account the story will lead to a fitting ending, sooner or later
2) Miura's a details maniac.
So in my mind there's no way he's doing all of his inhuman work without a plan about what direction his saga is going.
Fact is, this manga is so epic, and the long waiting between chapters makes it so much more like it's never going to end