Like many questions in running, when a runner asks me, "How many miles should I run?" my answer is either, "It depends," or, "How much time do you have to recover and rest today?"
If you want to stay healthy and run more miles, then you'll need to support that mileage with rest and recovery. One way to think about: It's not the stimulus of the workout that helps you gain fitness, but the hours and day or two following the workout where you "absorb" the training.
You probably won't be surprised to hear that the best runners in the world work out two or three times a day. But to support that training, they lie around and nap in between sessions. This is especially true of the Kenyan athletes; they train hard, then they lie down and rest before the next training session.
So what's the answer for you and your training? Well, if you're a stay-at-home mom with three kids and you can barely fit in an hour of exercise a day, you'll need to be careful with your mileage. Make sure that you have some recovery days during the week where you're cross-training, or perhaps doing something less intense, like a brisk walk.
If you're a single person who has a fairly normal 40-hour-a-week job, and you're fully committed to running PRs, then you can look at mileage as something you may be able to increase safely. So it depends.
More: How to Train for a PR
How Mileage Relates to Running Fast
If you want to run fast, you're going to need to do some decent mileage. Some coaches would tell you to do as much mileage as you can, and ignore the non-running work, such as General Strength and Mobility (GSM) and Active Isolated Flexibility (AIF). I'm not one of those coaches. I would rather a person come to me with a plan for how many minutes they have each day for their workouts. I would then ask them to do the Lunge Matrix, which takes three-and-a-half minutes, before each run, then commit 5 to 15 minutes after the run to GSM and/or AIF. This recipe keeps athletes healthy—from young moms with young families to runners in their late 50s who are running Boston-qualifying times.