I often hear people ask about weekly mileage, how many miles they should be running per week to get faster, etc. In training, there is a specific sequence in which adaptations occur. For example, you must have aerobic capacity built up to a certain level in order to produce benefits. That means that in the beginning of training, accumulating easy miles will occur in order to strengthen the aerobic engine so that your body can tolerate the training load as workout intensity increases. A key concept we know is that improving lactate threshold is very important for distance runners, but before working on lactate threshold, your aerobic base and neuromuscular systems must be established.
You must be able to tolerate the load in order to do it, recover from it, and then continue with training effectively. First, we must build capacity and repeat exposure. It doesn't mean volume, it means exposure. That means frequency. Run more times per week, not less times and more miles. Gradually increase the number of days you are running per week. Start running less miles each time you run, then gradually increase that so that you don't encounter form breakdown from fatigue. Increase your ability to tolerate the impact on your musculoskeletal system over time. When your form breaks down, you're practicing poor form and essentially bad skills.
Speed should happen year round in some capacity. Cutting speed work altogether will create more losses than cutting volume. Cutting frequency of runs will create more negative consequences than cutting volume. Running less miles, more times per week is more beneficial than running less times per week for longer durations. A lot of beginning runners want to run 3 or 4 days per week. It can be more useful to run 5 or 6 days per week with runs of shorter distance than cramming workouts and runs into 3 or 4 days per week. Frequency is important for improvement.
It's important to think about volume as a byproduct of the training, it is not a measurement of the effectiveness of training. The training program is created and volume happens as a result of the training and as fitness improves in order to maintain and continue adaptations. Volume is a very easy metric to track, but just because it's an easy number to see, does not make it the most important number to see. Volume is not the primary stimulus creating adaptations, it's the quality work you're doing with recovery that creates the fitness gains. Volume should not be the ultimate goal. What is making up the volume is more important. *Keep in mind, I'm talking about us regular people, not you ultramarathoners out there who get every mile you can get at any hour of the day.
As Steve Magness points out, total mileage can be useful as a means of measuring total stress. He also reminds us that the 10% rule is completely made up and not based in any science. It is an easy rule to follow for beginning runners who are just starting out, but it is not evidence based and every body operates differently. Having said that, of course increasing mileage too hastily can lead to issues. Increasing mileage can happen pretty quickly if you've already run that volume in the recent past (last 6-12 months). For example, post marathon after taking a couple weeks off with no running, you don't need to start over from 0 weekly miles and increase by 10%. That would take forever. But if you were running 60 miles per week you could probably return to 30, 45, 60 miles per week over a period of 3 weeks. Of course, this again depends on the person and what your body is able to tolerate. For instance, it would probably not be wise for a master's age runner to return to previous mileage that rapidly, or someone who has a significant injury history.
The bottom line is that mileage is very specific to each individual person. There are no hard and fast rules to follow. High mileage isn't necessarily better. You might not be able to achieve your running goals on 3 runs a week. You may be able to increase mileage faster than 10% per week based on your personal history. It's important to remember that you are an experiment of 1, and what works for one person cannot be carbon-copied to somebody else.