When preparing for a marathon or long distance run, it is typically recommended that you increase your running speed or distance by 10% every week. According to the experts, going beyond ten percent can increase risk of injury. What is omitted in this recommendation is the level of runners to whom it is addressed.
For example, an elite runner would never be able to increase running distance or speed by 10% every week or even every two weeks. Not only would his body not allow this, but it would most likely lead to injury. This is also true for a high-level runner and in many cases, a good runner, capable of posting respectable times.
Thus this recommendation is mainly for beginners, the only ones who can increase distance on a regular basis during an annual training cycle. After one year it is doubtful that such increases would — or should — be possible if the right training were done. For example, was the runner doing supplementary strength exercises to prepare the muscles for the greatest speed or distance?
Even for beginners, increases every week or every two weeks can lead to injury if continued over a long period of time. Your body will break down from the continual increases. It is often necessary to take a break or to stay on the same level for more than 1-2 weeks to allow for full adaptation to occur. You must give your body time to adjust and adapt to new distances or speeds.
Even more effective than regular increases is to do specialized strength and endurance exercises to develop the muscles to a point that allows you to safely and effectively make increases in distance and speed. Such physical preparation allows you to make increases safely because your muscles will be able to handle the additional stress.
A key point here is that the strength exercises most duplicate the exact joint and muscle actions that take place in the running stride. Merely increasing distance or speed of running with no other changes is questionable, especially when done regularly, because you will not be getting stronger physically.
Because of this you should always give yourself plenty of time in between increases if you truly wish to prevent injury. During this time you should continue to prepare yourself for the stresses to be encountered. Do not take anything for granted.
To learn more about specific strength exercises that duplicate the joint actions seen in the running stride, and that will allow you to make increases in your running in a safe and productive manner, see Explosive Running.