The Eagles- Napa's cycling club should be able to give you some idea of distance. I suggest you contact them. Great group- I've ridden with them several times.

On Saturday mornings they used to have a women's ride. I'm not sure if they still do. You might check that one out too.

I agree solo riding is fun, but at first I'd do some of the ride route you are talking about with others just to get an idea of what's available in terms of bathroom and food (should you need that) as well as water.