The post I was writing this morning degenerated into autobiography. I may be able to salvage some of it later, but let me just post the general idea as a kind of "thought for the day": You have to distinguish between the subjects you are merely interested in and the subjects you are an expert in.
The best way to define your expertise is to think about who your peers are. This, in turn, will define what discipline you are working in. It will condition your choice of department to work in, and the places you seek funding to support that work. It will also determine what classes you teach. In short, there are many good reasons to make sure that the way you construct your expertise does not stray too far from the subjects you are actually interested in. But it is also important to pursue some of your interests in a non-expert way, i.e., to allow yourself the pleasures an amateur's interest in topic. My current problem is sorting out what I am an expert in and what I merely find interesting.