Arthur Lydiard firmly believed that anaerobic workouts that produce lactic acid (i.e. interval training) during build-up of your aerobic base is counterproductive. "First thing: No. No. Never do anaerobic work in conditioning. Never. Ever." How long? "At least 12 weeks...The whole program takes 6 months." -Arthur Lydiard interview a week before his death.
A workout that Lydiard recommended, and I admit that I have never done, is short bursts of running (60 meters) with full recovery. According to Lydiard, these short speed sessions can be used safely during your aerobic build-up phase because you don't produce lactic acid. At the same time, your ability to run fast is maintained or even improved because you are recruiting fast twitch fibers, increasing power and efficiency.
Remember, the purpose is not improving anaerobic or aerobic energy systems. I'd start with 6 x 60 meters with full 1-2 minute recovery between each, then build to 10. This makes sense, especially for masters runners like myself who over time tend to lose speed with age.
A word of caution- build into this workout slowly, especially if you've been doing nothing but long slow running. Build your speed with each repetition, so your first is always slowest and your last 2-3 your quickest.