Core really isn't important to be honest. Your core gets a good workout from basically any exercise, including running and walking. The only real purpose to core workouts is to get super defined abs and obliques(they can increase stamina and endurance as well, but you are still probably better off running/biking in that regard), other than that you are pretty much better off ignoring them.
I mean you can throw a little routine in there for core workouts if you wanted, but I honestly wouldn't spend more than 5 minutes on it.