If you think no matter what action you are doing you are involving your core. It may only be a small amount but you are still involving it.
If you are running a lot then your core needs to be strong to keep you up right and so that you don't waste energy when you get tired. You core keeps all of these muscles working hard and it has a tough job. A simple way to strengthen your core without doing a physical workout is too sitting up straight. This helps keep your core strong. Your posture will also improve. If you think about it your tummy muscles are working so much harder to keep you up right than if you were to just slouch. If you want to go for a workout to work your core I have wrote a blog post on it called "super quick ab challenge" this has ways to make the moves easier or to make it more challenging for you. Go on, give it a try! What are you waiting for? http://behealthy2behappy.blogspot.co.uk/2013/06/super-quick-ab-challenge.html