I think they probably knew the beginning and the end on the first season, and settled on other things like time travel, Jacob and the monster in the second, which is where they started planting the seeds of what's happened in these last two seasons.
I believe you may be essentially correct but I think the time-line is different.
I would venture a guess that they didn't really start planning out the ending until
2007, when they got the network to commit to a specific end date.
I think that before that happened, they felt forced to make things up and stretch things as they went along because they had no idea if and when the network would end the show.
