Having a strong core is crucial to having good form in running. Your core consists mainly of your abs and back muscles. A strong core allows you to keep your back straight when your tired instead of being hunched over. And besides having a sexy six pack, strong abs help you pick your body pick up your feet when your tired and will keep you in a slightly leaning forward position while running.
Some of the most familiar core exercises are sit-ups and crunches. These are great to do, but not always enough to get the full effect of a workout. Depending on where you are at in your level of fitness depends on how much you need to do of each exercise. I would consider myself as very fit because I have been doing core work for over 4 years and have increased to a large amount of exercises. My regular core work involves 60 sit-ups, 30 push-ups, 50 V-ups (lay on your back and bring your legs to your chest and your chest to your legs to where you make a V shape with your body), 20 push-ups, 30 Bench-press crunches (crunches where your hands are vertical to the ground and you push upward while doing the crunches), 30 crunches with my legs in the air, 15 push-ups, 20 large and small Switches (lay on your back and move your legs in a scissor-like motion), and a 1 min - 2 min Bridge (also called plank, where you are in a push-up like position, but your elbows are on the ground with your forearms supporting them. If you hold this position with good form you will feel your muscles straining to hold up your body). This takes me about 10 min to complete and I do this anywhere from 2 - 5 days a week and sometimes do diffrent variations.
For beginners I would start out with 20 Sit-ups, 5 push-ups, 10 V-ups, 5 push-ups, 20 regular crunches, 5 push-ups, and a 30 second bridge. Do this 3 times a week. I would not increase reps until you become comfortable (feels tired but not exhausted) at this amount. Once comfortable, slowly increase to where your body struggles and stay there till your body becomes comfortable then repeat the process.
If you find yourself in the middle between beginner and advanced, start with 35 Sit-up, 10 push-ups, 30 V-ups, 10 push-ups, 15 bench-press crunches, 15 cruches with legs in the air, 10 push-ups, 10 Switches, 45 second Bridge. Do this 3 - 4 days a week.
Very soon I will post a much more detailed explaination of the exercises with pictures so it will be easier to understand.