So, I highly recommend adding two or three core workouts to your weekly workout regimen. You can mix core exercises into your other workouts or you can have separate core-specific workouts. It's all good. Remember, the core refers to the entire torso (front, sides, and back). So, mix it up. Don't do just crunches. Just doing crunches can actually cause an imbalance in your torso. It can even pull you forward if you over develop the abs.
My friend and fellow blogger from Down Under, Amelia Burton has put together a great core/oblique workout. It's perfect for runners. One thing many runners lack is muscular endurance in their lateral muscles (upper and lower-body). Increasing the muscular endurance of your obliques (the muscles along the sides of your torso) along with other lateral stabilizer muscles in the lower body will not only help with stability and posture, it will also help your body react more quickly if you trip or stumble and get out of your normal forward movement pattern. That means you'll be more apt to rebound and prevent a fall or lessen its severity.
So, check out Amelia's awesome workout and WORK THAT CORE!