Ask HN: How do you market a productivity tracking app?

I'm building Mastery, a habit tracking app for the iPhone. I've also setup a bare minimum landing page (for free) for people to signup. I've got 13 signups in about a week.

However, I'm struggling to define my target user group. They can be anybody trying to do better in life. Starting from indie hackers who want to code daily, fitness enthusiasts who want to hit the gym daily, to bloggers who want to write daily.

It's unhealthy to have such a broad, generic and ambiguous target group. Need advice.