Active rest is a highly effective modality, so on your non-running days you can head out for a long walk or take a yoga class instead of sitting on the couch.
Cross-training gives you the ability to develop your aerobic (cardiovascular) system without the repetitive pounding brought on by adding extra miles on the road. Jump on a bike or into the pool, and give your bones and joints a break to help develop resilience in the long run.
Strength-training is also key for preventing injuries, helping to correct muscular/postural imbalances or increasing the effectiveness of your run-specific programming. This is why a good coach is a wise investment; they can ensure you are maximizing all these different training modalities and guide you through a comprehensive, holistic training regimen.