Upper body reinforcement is essential for runners as well. You have to balance everything for improving. What is really not important for a runner is to oversize or hypertrophy your arms and biceps doing repetitions with enormous weights... but doing so with a light weights is important for improving body coordination and get a balanced structure.
I always do Core training, once a week at least.
Well done mate!
I'm probably never going to be super muscly, but I'd like to tone up a bit. It's all a compromise of time and results.