To Improve, Do I Add More Mileage or Speedwork? | For Beginners Only

To Improve, Do I Add More Mileage or Speedwork? | For Beginners Only: "I finished Couch to 5K in October, and since then I've kept running 3 miles, three times per week, plus I've built up to one 5-mile run per week. I've run in two 5K races, and I'd like to get faster. Over the past month, I've dropped about 30 seconds per mile off my pace without really doing anything. How long can I expect my pace to improve just by adding mileage and gaining experience? How soon can I start speed work? Thanks, Brittany"