The vast majority of running injuries are linked to overuse injuries. Your body needs time to adjust to the activity and the 10 percent rule is an easy and safe method to make sure your body adapts gradually. It is a tool for you to calculate how much you can increase your training load each week. The formula takes into account both time and self reported intensity.
Here is an example of how to use it:
Day 1: Easy 50 min run - intensity 4 (on a scale from 1-10).
Day 2: Intervalls, harder, but less time. 36 minutes and intensity 7.
Day 3: A short run, 30 minutes and intensity 5.
Total 602 points
Next week: Do not increase more than 10%, which means you can only add maximum 60,2 points to your weekly calculation. As an example, 10 min running with an intensity of 6 out of 10, equals 60 points.
Don't forget this FACT: it is easier to prevent than to repair an injury!