I'm wondering how whether core training is important for training for running a marathon? Is it worthwhile? Or should I not waste my time?

I think so,yeah.

Core training strengthens the oblique. abdominal muscles, and lower back. By and large, running does nothing to strengthen those muscles. But those muscles are important to runners.

We run from the hips and thighs, but using those core muscles and keeping our bodies upright is crucial to running.