Using Mileage for Running Workout

I have changed my cardio workout for running to reflect mileage rather than time. I am hoping to encourage increasing my speed this way as running 20-30 minutes, regardless of the mileage, hasn't been helping me increase my speed. Plus, running too slow of intervals hardly is helpful to my overall goal of running an 8:30 mile. So, I am going to work on bringing down my mile speed and work on completing 2 miles in the shortest period of time I can as long as I have an adequate warm-up and cool-down. This would mean, I could get down with my cardio workout in 15-20 minutes with a problem. I will work on this 2 times per/week and then maybe work on a longer run on the weekend. I will still need to monitor the calves but I need to start pushing the speed if I hope to run 8:30 easily and consistently by December.