Generally when I start a fic it has a planned beginning and end, and it's the middle that always gives me grief :D Writing an arc with a start-climax-finish is quite different to writing an ongoing story, both with their good and bad qualities. But to know that there is a definite end, and to be working towards it, can be a little mind-boggling sometimes. Especially when you actually reach it.

Torchwood totally dazzled me away from a major HP work-in-progress too, so I feel you there.

Or write the ongoing adventures of Captain & Mr. Harkness-Jones?

The sad thing is I can think of like four fics off the top of my head that you might be referencing here.