One of the unresolved questions among physiologists and runners concerning aerobic training is: Is it more effective to tax the upper limit of VO₂max with a low volume of interval training at 100% VO₂max, or run a greater volume at a slightly lower intensity, such as 85-90% VO₂max?
Over the years, I have had success training runners with both approaches. However, for speed-type runners who have a high proportion of fast-twitch muscle fibers, running less volume at 100% VO₂max seems to be more beneficial. For endurance-type runners who have a high proportion of slow-twitch muscle fibers, running a greater volume at 85-90% VO₂max (which corresponds to lactate threshold intensity for very good runners and slightly higher than threshold intensity for everyone else) seems to be more beneficial.
Know whether you are a speed-type runner or an endurance-type runner (you can find out through careful attention to your training and racing), and then skew your training to your strengths. Not every runner needs to do every type of workout.
Speed-type and endurance-type training programs are available at drjasonkarp.com/trainingprograms