Distance
As your technique improves, you'll be able to run farther with less effort. The way to determine when to increase your distance is by observing how long you can successfully hold your running technique.
When you're ready to add distance or time to your runs, do it in small increments. Even a couple of minutes or a few tenths of a mile extra will improve your conditioning over time.
The key is to be mindful of how your body feels while you're running additional distances, and afterwards. The same stresses that, in moderation, help our bodies adapt and grow stronger can also cause injury if overdone.
When your body can't keep up with the adaptations, you may stop improving, become overtired, or even injured. Steadily build your distance through persistence and patience, and enjoy the process.
Speed
Finally, what you've been waiting for. As you are able to hold your running technique for longer periods of time over greater distances, you will develop increased speed at a lower exertion rate.
As you get faster, you'll notice that your strides automatically lengthen. To prevent yourself from over-striding, make sure your feet stay under your center of mass with each step. Also, make sure your cadence remains between 170 to 180 spm, even as your stride length increases.
This limits the time each leg has to spend supporting your body weight when you take a step. A metronome is a very useful tool that will help you keep a sustainable pace as you increase your speed. We don't recommend many gadgets for running, but this one can benefit you enormously.
The second title to the Chi Running book reads: "A Revolutionary Approach to Effortless, Injury-free Running", which is a bold statement. Turning your running into a "practice" with the goal of becoming effortless means that every time you go out for a run, you're working on creating better results by doing less. That's the real work.
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