As a runner, your base is your aerobic fitness. The stronger your base, the more solid of a runner you'll be and the more prepared you'll be when it's time to do speed work, whether you are recreational, elite or somewhere in between.
Deciding How Much to Run
How much mileage do you need to run to build an adequate aerobic base? The answer depends on a number of factors, including your genetic ability to adapt to aerobic training, the amount of time you have to run, the specific racing distance you're training for and your running goals.
Interestingly, there isn't a proportional relationship between weekly mileage and racing distance. In other words, if you're training for a half marathon, don't expect to run half as much as if you're training for a marathon.
Even though the marathon is twice as long as a half and more than four times as long as a 10K, marathon runners don't run double the weekly mileage as half marathon runners and don't run four times as much as 10K runners. Even for shorter races, you need to run a lot. The best 5K runners in the world run almost as much as the best marathon runners.
The reason for this is that any race that takes longer than three minutes to run is primarily influenced by your aerobic system. The shorter the race, the more critical anaerobic training becomes. But for all distance-running races, aerobic metabolism is king.
The best way to determine how much aerobic training you need is to slowly and systematically increase your mileage each month and year, taking care to note how you respond to the training stimulus.
More: Distance Running: How Many Miles Should You Run?
If you're already running more than about 40 miles per week, don't increase your mileage unless your prior training and racing experience gives you reason to believe that you will continue to improve with more mileage. If you haven't reached a plateau in your performance at 40 miles per week, there's no reason yet to increase your mileage to 50. If you're running 20, chances are you'll get much better running 30 miles.
Despite what many runners believe, more is not always better; more is only better if you continue to adapt to more. In general, most runners do adapt to more.
However, increasing your running mileage should still be done systematically and with reason behind it. Some runners thrive off more speed work and are better at short distance races compared to long distance ones. Others, especially females, tend to adapt to and fare better with more aerobic training (due to the estrogen-enhancing use of fat during exercise), as long as precautions are taken so they don't get injured, such as consuming adequate iron and calories and paying attention to menstrual irregularities.
More: How to Use Your Menstrual Cycle to Become a Better Runner