Sometimes it's not the quantity that you write in a day - it's that you're writing. But even when you're not writing, you're writing - just in case you wondered.
Yesterday I forgot my Dana while I was at my day job, so I couldn't do any work on my current books. However, I did a lot of thinking about my characters and my career, and things I'd like to see myself accomplish. While I haven't done as much this year as I'd like, I am pretty comfortable with where I am at for the moment. (Talk to me my December, and I might be mad I didn't get further this year!)
Just remember to get yourself in front of that keyboard daily if you can, and as frequently as you can if not daily. Writing gets rusty when not used regularly, like any other talent. But realize that as a writer, your downtime away from the keyboard is often spent subconsciously working out details, dialogue, scenes, and conflicts in your book. Then, even if it's ten minutes, get back to the keyboard and write out what you worked on in your head. As I said, you need to do the physical writing regularly to keep your skills honed.