The ability to tolerate impact is one of the most underestimated factors when considering run training. Too often we focus on energy systems and forget that the ability to tolerate impact is usually the limiting factor when it comes to run performance especially over longer distances. The ability to tolerate impact is also incredibly important from an injury prevention standpoint.
Progressive training load is essential to avoid running injuries. Running is incredibly high impact. You are basically taking your entire body weight, throwing it up in the air and landing on one foot and you are doing this repeatedly for long durations. Whenever you land all of that energy is absorbed and presents in your muscles, bones and ligaments. Your body needs time to experience loading and then recover and get stronger. If you do too much too soon you will definitely acquire and injury. Progressively increasing load based on your running efficiency, experience and body type is the best way to avoid issues.
Once your body has reached a point where it can handle the running impact load comfortably for extended periods of time you can then start challenging yourself with higher intensity and volume but this must also be progressive in order for you to have success. Long runs and higher intensity runs both present the body with larger amounts of impact load. If you have been progressive with your training and you have refined your running technique so you are biomechanically efficient you will limit your risk to injuries that result from repetitive impact load.
Including some downhill running at faster paces and on gentle downhill grades is a great way to exaggerate loading and help you increase your tolerance for impact. Workouts that include downhill running should be done with caution and only after you have built up your running to a point where you can handle the extra loading.
When you are considering your run training focus as much on your ability to handle impact as you do on building up the appropriate energy systems needed for success. Do this through progressive loading, limiting volume and intensity until you are ready, including some downhill running and making sure your technique and biomechanics are on point!