Writing long form narratives are so much about building a story, building characters, building a world for them to inhabit--real or imaginary. While I know how the story is supposed to develop and that I need to get my heroine from point Q to point Y, so that X can happen, it is sometimes very hard to do so. It's hard to slow down. It's hard to spend the time to see the scenary as she sees it, experience the weather or talk to an old friend like she does. In order for the plot to proceed, these things must happen at a precise speed with enough detail to enrich the place but without so much that it bogs down. It can be a very fine line.
I find myself, lately, concentrating on only one scene a day. I really try to flesh it out, even if it is only a short chat with a minor character that doesn't really help get my heroine to the next plot point. Part of why I like having fictional lives dominate my brain is so I can watch them develop as a human. If I skip the detail writing and concentrate only on forwarding the plot, so much of why I like the person I'm writing is lost.
Do I get lost in too much detail? I'm sure I do, all the time. But that is what a first draft is for, to really dig deep into the world you are writing. My red pen will definitely be well-used once the first draft is finished. Only 60,000 words or so to go.