One of the biggest mistakes runners and coaches make is not considering the demands of the race and not being specific enough in their training to meet those demands.
If you don’t already have a strong background of speed, try devoting a significant amount of time to becoming a faster runner first and then training the endurance of speed.
VO₂max should be targeted after improving anaerobic capacity. Even though VO₂max is considered an aerobic factor that represents the maximum capability of your cardiovascular system to supply oxygen to the working muscles, it is often unrecognized that the running speed at which VO₂max occurs includes a large dependence on anaerobic metabolism. In other words, when you rev your aerobic engine as fast as it can go, your muscles also rely on anaerobic metabolism to run at that speed.
Research has shown that the amount of time athletes can sustain exercise at VO₂max is positively correlated with their anaerobic capacity, and that sprint-interval training improves VO₂max, supporting the significant role that anaerobic fitness (speed) plays in VO₂max and distance running performance.
The improved anaerobic fitness gained from speed training also makes the more metabolically demanding VO₂max workouts seem easier. Jumping into VO₂max workouts without first training speed makes the already hard VO₂max workouts even harder because of their aerobic and anaerobic metabolic stress.
VO₂max should also be improved before focusing on improving the fraction of VO₂max that you can sustain, which is reflected by the acidosis (lactate) threshold, as well as other metabolic factors. In other words, build as large of an aerobic engine as possible, and then build your sustainability of the engine — 80% of a Ferrari is faster than 80% of a Mitsubishi.
To be the best runner you can be, make your VO₂max as high as possible (and increase the speed at which you’re running when you’re running at VO₂max) and be able to sustain as high of a fraction of your VO₂max as possible (which is reflected by the acidosis threshold and running economy). If you acquire both those traits — a high VO₂max and a high fraction of VO₂max that you can sustain — your marathon and half-marathon performance will reach a much higher level.
— from The Endurance of Speed