Writing "fillers" in long adventures is pretty hard, yeah. It's a lot easier in enclosed spaces scenarios, where while it may be long, the tension rises easily and things may go wrong in many many ways. When it's travelling and other more quiet, relaxed or positive environements, it becomes harder.
From my experience, it's easier if you have the main idea of the story, then you detail the beginning and do a pretty detailed outline of the ending. Then, you decide about a couple of events or situations you'd like to happen - may they be important for the story, the characters, or simply funny or interesting, even if unrelated with the main story. And you decide about when to put these along the timeline, trying to make regular segments in between. And then it's easier to fill the segments with more random stuff, and sometimes linking two of the events together makes it even easier to come up with the things that happen in between. And after that you can polish the ending part - better not to do it before, since the filler events you come up with may end up being more important than you thought and influence the ending.
Well, at least that how I work when I write stories haha!