If I were you, I would sign up for a 5K (3.1 miles). Most people that train for running follow a weekly schedule where they increase their speed and distance, usually running 4 times per week. Some workouts are designed to increase speed (usually shorter distances) and some workouts are designed to increase distance (slower, but longer distance). Take a look on the web and search for 'beginner training for 5k' and you will find lots of options.
For my first 5K I set of goal of 30 minutes and worked to increase my speed until I could run at that speed for 3.1 - 4 miles.
Tip 1 - run and train outside. While treadmills are nice, they will never prepare you for running on the road. By running outside you get used to the hardness of the pavement and the ups / downs of the hills. Use to trace your route and it will tell you how far you ran and what your average speed was.
Tip 2 - Like I said in my earlier post, go ahead and sign up for a race. Once your committed, you will train harder.
Tip 3 - Give your body a day to rest between running days, especially in the beginning. Use Saturday or Sunday as your 'long' running day where you run slow and far. Before the 5K you should be able to run about 4 miles without stopping / walking.