Who Are You Writing For?

I have six young adults and one teenager under my roof at the moment, plus a husband and a dog. Don't ask me how this happened. I'm not really sure. My daughter caught me in a moment of weakness, and next thing I know, she's bringing home all these college kids...you'd think Bermuda was a nice place to visit or something.
Anyway, it's been fun, despite the fact that we have no food left in the house.
They were talking the other night about movies and what they liked, didn't like. I found it interesting that some of the movies my husband and I have recently enjoyed, they hated. Which got me thinking.
Who am I writing for?
No, I'm not talking about God. For me, that goes without saying. But who do I think my target audience will be if my books ever hit the shelves of a Barnes and Noble (unlikely since i hear they're going out of business), okay...erm...let's say Amazon then. If you've ever had to prepare a book proposal, this is one of the questions you have to answer. I always give a wide variable, women between the ages of 18 - 80, which probably sounds ridiculous, but I honestly have no idea. I'm not published yet so obviously I have no figures to draw from. I would expect my readers to fall into this category because I don't know a whole lot of men who read romance, but it's just a guess.
I think the only way around this right now is to write what I know I would enjoy reading. That sounds kind of simple. Obvious even. Like really, why would I ever write Amish Fiction? Well, because Amish fiction is a hot commodity. So there comes another question.
Do you write for the market or for yourself? Why do you write?
No, I don't want to hear, "I write because I can't not write," or "If I didn't, I would die." Please. Can we be a little more creative?
My answer is simple. I write because I enjoy it. I really do.
Yes, I grouse, I get discouraged and I do have days when I just want to walk away. But for me, that's the thing. I can walk away. Nobody is forcing me to sit at the computer and come up with something worth publishing. There are a ton of other things I could be doing with my time, probably a lot of things my husband wishes I was doing...but I don't know...this writing thing is kind of fun. And now that I'm actually getting somewhere, I hope, I want to stick around and see how it plays out.
Which brings me back to the audience I'm writing for. I'm not sure I'll really know who they are until I get published. I know people who choose a few select few as a 'target audience', and send their manuscript to them for feedback. I've tried to do this, but it's extremely difficult to enlist people who can devote the time to actually reading a 300 + page novel all the way through and give you honest feedback afterward. (If you are interested, let me know!). So really, I'm back to guessing. Which doesn't help my someday publisher much, does it?
So I'm curious, those of you who have already been published, how did you get around this? Was it guesswork or did you have some concrete evidence to present to your publisher as to who would be most likely to pick up and enjoy your novel?
Enlighten me!