When training for trails the first thing to consider is your running mechanics and technique. Your running style will differ somewhat from road running, with the changes to surfaces and the undulating terrain. A good way of looking at running on trails would be comparing road and offroad vehicles. If a non-offroad vehicle drives over difficult, uneven tracks then there is a good chance it may breakdown. This is the same for running on trails, if the body is not conditioned to run on trails then there is an increased chance of getting injured.
