It makes a lot of sense to strengthen your diagonal stomach muscles, as you twist with every step. Especially for running in rough terrain core strength is essential.
For a relatively competitive runner I suggest 2 trainings/week of about 20 minutes each.
Type of exercise:
http://www.gc-slr.de/fileadmin/gc-slr.de/downloads...
The document is in German, but the images should explain what to do.
Important is that you keep your pelvis stabilised. That means that you don't tilt even when lifiting one leg or the other. The muscles that stabilise your hip/pelvis are the core muscles that you are training. In general you should have one straight line from ankles to shoulders when stabilising on your elbows or on the side. Try to check in a mirror.
It might also make sense to train with someone who has been doing core strength for a while, then you can control each other when doing the exercises.
Hope that helps!