When I began my first science fiction series, the first novel of the space-opera was a self-contained story, quite straightforward to write. The second felt more difficult, and I thought the third would be the last, but the story arc spilled out and I wrote a fourth (and last!) of the Jules-Verne saga series.
It felt like my training running hills. The first time is easy, but by the fourth time, my legs were almost quitting under me! That fourth and last novel of the series was the most difficult to write, since I had to wrap up the leads to complete the neat story arc.