Ever notice that some runners can add mileage to their base without much apparent effort or problem, while others struggle with fatigue and injury? There are many factors in these differences but one easy one to control is how well an athlete sticks to the Ten Percent Rule during a training cycle.
Simply put, the Ten Percent Rule is a guideline for building your mileage consistently and safely. You only increase your long runs by 10% in any given week and you only increase your total mileage by 10% per week. The slow and steady pace of increase you get from following the Ten Percent Rule gives your body time to adapt to and recover from the stress of additional miles--you push yourself just enough to improve but not so hard you overtrain.
So, if you're carrying about 15 miles a week and your long run is three miles, the first week of mileage increases you can safely bump your long run to 3.3 miles and your total miles to 16.5. This gives you a 10% increase in both your long run and total miles. The next week, you add 10% onto these totals, so your long run is about 3.63 miles and your total miles come to 18.15. The incremental increases are steady and slow, but slow is safe. If you plan your training schedule out far enough, you can prepare for very long distances this way.
Now remember, the Ten Percent Rule is about setting safe limits, not meeting goal mileage. In our training plan above, you can run a total of 16.5 miles in your first increase week, but you don't have to. You can just focus on increasing your long runs by 10% and be casual about adding another short run or increasing some of your other workouts to make a total of 16.5 miles--maybe 15.5 or 16 is enough. This is important to remember, especially if you are recovering from an injury or illness or have other concerns about ramping up too quickly.
Also, most of us have a natural limit to the amount of miles we can run per week. Reaching to expand those total distances can make us unhealthy or unhappy. That's 100% important to recognize about yourself. You can run 20 miles a week and be completely maxed out your total mileage, but still want to redistribute you runs so that some get longer and others get shorter. Use the 10% rule to increase your long run, but keep your total mileage steady by shortening others.
There is no foolproof way to stay injury free while you're increasing your running miles, but having a useful framework to make slow and steady improvements may help you to feel healthy and strong while you take on the challenge. The Ten Percent Rule is one of several useful tools to help you asses your training plans and make this process fun, manageable and safe.