Jason Fitzgerald is a 2:39 marathoner, USA Track & Field certified coach, and the founder of Strength Running where runners go to get faster and prevent injuries.

Have you heard of the 10 Percent Rule? Most runners have — and swear by the collective wisdom that you should increase your mileage by no more than 10 percent each week.

But it’s flawed. The 10 Percent Rule is, in most cases, incomplete, and blindly following it could result in not building the fitness you need to reach your running goals. Worse, it could leave you injured. There are many situations when you should add far less than the recommended amount — and sometimes far more!

But before we talk about building mileage, let’s remember why we log all those miles in the first place. Why is it so important, anyway? The main reason is because it indicates your workload, or how much volume you’re running every week. There’s no better indicator of success as a distance runner than your mileage. Higher volume training results in more endurance, allowing you to hold onto a faster pace for a longer period of time. And that’s the stuff that personal bests are made of!

So if mileage is so important (particularly high mileage), how do we do it right? When it comes to running, there’s no such thing as a one size fits all plan. Here’s how to modify the 10 Percent Rule to better fit your training.

When It’s OK to Exceed the 10 Percent Rule

Before you break out the calculator, author and ultra-marathoner Matt Frazier recommends sticking with a simple guideline that’s easy to remember. “When your weekly mileage is neither very easy nor very difficult for you to maintain, that’s when it’s most appropriate.”

But like any rule, the 10 Percent Rule can be broken. Frazier explains, “You can break the 10 Percent Rule if your weekly running volume is below your baseline mileage — an amount you’ve maintained for several weeks comfortably and without injury.”

Many runners are comfortable at about 15 to 25 miles per week. So if you’re only logging 10 miles this week, it makes sense that you can build your mileage more aggressively until you get to this “baseline” level. Your next three weeks can increase from 10 miles to 13, then 16, and finally 18 until you’re within your baseline weekly mileage range.

When to Add Less Than 10 Percent

“Very few runners will have the ability to increase their mileage at the 10 percent rate for more than a few weeks at a time.”

If you’re a runner who’s comfortable running about 20 miles per week, you should be more cautious when your weekly mileage is over 20 miles per week. Past this level, you’re in uncharted mileage territory. This is where the 10 Percent Rule is too aggressive and a more conservative approach is best for injury prevention.

Any time your overall mileage is higher than usual level, it’s best to play it safe by adding less than 10 percent per week. Otherwise, you may find yourself becoming overly fatigued, over-trained, or hurt.

As Frazier says, “Very few runners will have the ability to increase their mileage at the 10 percent rate for more than a few weeks at a time.”

Two More Mileage Strategies to Stay Healthy

When your mileage does increase above your baseline, there are two helpful strategies to keep yourself healthy and running strong.

The first is what’s called an “Adaptation Week” — or even more simply, a week of training that is repeated a second time. Doing the same mileage and workouts allows your body more time to recover, adapt to the new volume, and get stronger.

One of the many flaws of the 10 Percent Rule is that it tells you to increase mileage every week. But the body doesn’t necessarily adapt that quickly; it needs more time to adjust. Adaptation weeks can help you run more mileage, but without the risk of being chronically injured.

“Recovery weeks remind us that our bodies are not machines. We require rest and time to adapt…”

Periodically, it’s also helpful to run a Recovery Week, a short stretch of time where your training is reduced. These “down weeks” typically have less mileage, fewer fast workouts, and the workouts themselves are easier than those you’d normally do. So if you normally run a five-mile tempo run, you can cut that workout to only three miles.

Recovery weeks remind us that our bodies are not machines. We require rest and time to adapt to faster workouts, higher weekly mileage levels, and more intense training.

When in doubt, it’s helpful to remember that sound training is usually two steps forward and one step back. Without that necessary step back, we often fall prey to overuse injuries and ruin the consistency that’s necessary to get faster. Using these principles and better understanding the many nuances of the 10 Percent Rule — rather than plowing right through — can help you build your mileage safely so you can keep moving forward.