Among the most important physiological adaptations to aerobic training are increasing the number of capillaries bringing oxygen-rich blood to your muscle fibers, increasing the size and number of mitochondria in your muscle fibers, and increasing the ability of your muscles and liver to store glycogen. Aerobic training also improves your ability to recover from higher intensity training, such as VO2 max intervals and speedwork.
You can build your aerobic base by increasing your running mileage, and/or doing more aerobic crosstraining. Unfortunately, some running writers have spread the misconception that increasing mileage produces little benefit to running performance. How then to explain that the world’s best distance runners typically put in 120 to 160 miles per week? Rather than mileage for mileage’s sake, they train this much to reach their full potential and to gain an edge on runners who cannot handle that volume of training. Racing performance clearly improves after a sustained increase in mileage (and the longer the race, the greater the benefit), but, as with other aspects of training, the incremental improvement follows the principle of diminishing returns.
Enthusiasm to increase your training volume needs to be tempered by the reality that running more miles increases your likelihood of injury. Your personal training and injury history is invaluable in determining how much mileage is optimal to help you achieve your running goals. The objective is to increase to the point at which the incremental gains become outweighed by the likelihood of injury. Of course, you do not know the precise level at which this occurs, but using past experience as a guide, you should be able to estimate fairly accurately. For example, if you have been able to manage 45 miles per week, but broke down last year after several weeks of 60 miles during a marathon buildup, the challenge is to modify your training and lifestyle so you can handle 55 to 60 miles per week.
Several effective strategies can help to minimize your risk of injury while increasing your mileage. It is wise to avoid high-intensity interval sessions, because these workouts add considerably to the overall stress and recovery requirements of training. Tempo runs and strideouts, however, do not tend to lead to lasting fatigue or sore muscles, thus are safe to include as you increase your training volume. When you are building up your mileage, it is particularly important to train on soft surfaces to reduce impact forces and to be sure that your running shoes suit your needs and are in good repair. You should also pay attention to lifestyle factors such as getting adequate sleep, eating enough carbohydrates, and staying well-hydrated. Finally, listen to your body, and if a muscle or tendon is tight or painful, back off for a few days until it recovers.
How should you increase your mileage?
Although you can increase your mileage dramatically over a few years, increasing too much at once is almost certain to lead to injury. This next piece of advice makes me nervous because every runner is an individual: most runners can handle an increase in training volume of 10 to 15 percent over two to three weeks. For example, if you have been running 40 miles per week, you would increase to 44 to 46 miles per week for at least two to three weeks, before increasing your mileage again. Some runners will find this guideline too aggressive, so you should pay close attention to your body’s feedback to determine what it can tolerate.
When upping your mileage, it is more effective to increase the distance of your runs first before increasing the number of runs you do per week, because the endurance benefits of one 10-mile run are greater than the benefits of two runs of 5 miles. The longer the distance you are training for, the more this principle applies.
Another way to improve your aerobic base, with less risk of injury, is by aerobic crosstraining. Any form of aerobic training causes "central" adaptations that improve your cardiovascular system (e.g., ability of your heart to pump more blood per contraction), and "peripheral" adaptations within the working muscles. Therefore, all aerobic training is beneficial, but the closer the activity simulates running, the more transferable the adaptations in the muscles will be. The types of crosstraining with the greatest cross-over to running include: deep-water running, elliptical training, stair climbing, cross-country skiing (classic technique) and, to a somewhat lesser extent, cycling. Crosstraining is particularly important if you have a propensity toward injury, or if you live where winter or summer weather interferes with your training.