I must really "kill" that hypothesis that you get more aerobic fit the more mileage you run.There is absolutely NO evidence that so would be! There is probably a limit for every individual how much mileage is needed to reach the aerobic highest capacity. Also to reach optimal aerobic capacity is not only a question of running high distance mileage. Even the paces that we use to call anaerob or around the threshold will contribute to the aerobic power. Yuki Kawauchi is a great example that shows you can reach your optimal aerobic capacity on just singles in training and not very high weekly mileage.