I recommend three exercises. The first is a straight crunch. Lie on your back with knees bent, feet on the floor, and hands resting on your chest. Curl your head and shoulders about a foot off the floor, pause for one second, then return to the floor. Begin with five to 10 of these.
Start the second exercise in a posture that looks as if your chair has fallen over backward with you still in it. Lying on your back, lift your legs while bending your knees at a 90-degree angle, so your lower legs are parallel to the floor. (Put your feet up on a chair or against the wall if this makes it easier to hold the position.) With your hands on your chest, do the same curl as in the first exercise. Again, begin with five to 10.
The third exercise has the same starting point as the second, but adds a twist. As you curl up, bring your right shoulder toward your left knee, and back down. Then curl forward again, this time bringing the left shoulder toward the right knee. Begin with five to 10 for each side.
Do these exercises after every run, and add five reps of each exercise weekly until you're doing 50 of each. Do not do them before your run, because this may result in abdominal cramping.