Balancing Run Volume and Intensity

From MarathonPerformance.com

Since the only way to truly maximize running mileage is to forgo high-intensity training, I believe that overemphasizing mileage is a mistake. Most runners will get the best results by finding a balance between quality (intensity) and quantity (volume). So the adaptive running approach is to do as much running at various faster speeds as you can do without seriously limiting the total running volume you can absorb, and to do as much total running as you can do without seriously limiting the amount of high-intensity running you can absorb. Naturally, the precise formula is different for each runner, and finding it requires experimentation.