Excerpted from Running With Joy:
In high school, I was the mileage king. I loved the sense of accomplishment I felt when I added up my weekly mileage. Sometimes I would even sneak out for an easy extra run so I could hit a nice round number. But now I have learned that running a certain number of miles a week doesn’t necessarily mean I will run faster. Running is more art than science. Everyone has unique and changing needs. The only way to know exactly how much is enough but not too much is to experiment and get to know your body as well as you can.
The ideal mileage is the mileage that is right for you. More is not always better. I recommend a gradual progression, trusting that your body will tell you when you have crossed the line and gone too far. While increasing mileage, run on soft surfaces, such as grass and dirt, whenever possible. Also, when increasing your mileage, alternate longer and shorter weeks.
I am always reluctant to tell people how many miles a week to run. I don’t even keep track of my mileage (except when I was writing in a journal heading into Boston last year, and even then I never added up the weekly mileage). I get more confidence from hitting big workouts at specific paces, such as a 15-mile temp run at five-minute pace at 7000 feet over rolling terrain. My coach’s approach has always been to hit the big workouts and then go as short as I need to the following day. I recommend focusing on the workouts and filling in the easy days with as much volume as feels right for your body.