Generally, I think of research and writing as two distinct activities. Research, the collection and assembly of evidence, leads to writing, the arrangement of that evidence in narrative form.
Of late, I've felt that writing actually extends research. The overlap of assembly into outline into letting the argument of the narrative take over have made all of one piece. The writing part exposes the holes in the research, and the research drives the narrative, which turns the whole process into a loop.
The days of greatest fun come when I flip back and forth between the two, sometimes ranging wide and sometimes training microscopic attention on some puzzling detail that does not quite fit like it should. Those days may not end with a higher word or page count, but they give the words on the page greater depth. Even if they do not accomplish that, at least not obviously as of yet, they ensure that I cover every line of inquiry, they eliminate blind spots from which stupid mistakes or obvious oversights surprise me.
This has been one of those fun days, and I don't yet want it to end.