Runners often become the butt of a very similar joke. They run. They get injured. What caused the injury? Running too much, they are told. How can they prevent the injury from recurring? Run less, they are told. It sounds so reasonable. After all, if you do not run a single step, you cannot ever suffer a running injury. If you run just 100 yards a day, it is still very unlikely that your running will cause any sort of breakdown. If you run 5 miles a week, the chances of a running injury cropping up are a little greater. At 10 miles a week, the risk is greater still, and so forth. In this way of thinking, all running injuries are almost definitionally caused by running too much. But there is one major problem with this definition of running too much. A runner who gets injured when running 20 miles per week this year might not get injured when running 20 miles a week, or even 30 miles a week, next year. It happens all the time. Injury is the primary training limiter for most competitive runners. We train as much and as hard as we can without getting injured with unacceptable frequency. As discussed in Chapter 3, learning how much and how hard you can train without breaking down all the time is a major part of the process of developing your optimal training formula. But interpreting injuries is tricky. It’s never as simple as discovering that, for example, you never get injured when you run 59 miles per week or less and you always get injured when you run 60 miles per week or more. Very few competitive runners find that they get injured only when they run more than a certain amount. And limits change. I used to run 60 miles per week and got injured all the time. Later I ran 80 miles per week and got injured less often. Refusing to accept a 60-mile limit motivated me to find ways to run more without falling apart, and that quest made me more durable. Thank goodness I didn’t listen to those who said I was doing too much! Because it is difficult to define limits through injuries, and because limits can be increased, it makes more sense to use factors other than injury risk as the primary determinants of how much and how hard you train. Namely, you should try to run as much as you feel you need to run to achieve your goals and/or as much as you feel you should be able to run healthily. Table 9.1 presents normal running mileages for different categories of runners that were developed by Brad Hudson. Most runners naturally choose running mileage targets that are appropriate for their category, and most runners can hit those targets without unacceptable injury frequency. Nevertheless, there is a small chance you will never be able to healthily run as much as you want to. If you keep hitting your head against a wall, eventually you will have to read the writing on that wall and set your target lower.