Running provides a lot of health benefits but for overall fitness it is advisable to do different forms of exercise. i.e. cross train. Supplementing running with cross training can provide improved fitness, build strength and flexibility, prevent injuries by correcting muscular imbalances, quicker recovery and prevents burnout. You could pick from a variety of things like cycling, swimming, playing a sport, yoga, strength training and others. Every runner is different and preferences are different. I
find that runners are either strong and inflexible or flexible but weak. If you can understand what category you fall into, you will be able to pick the right form of cross training more effectively. For a runner who is strong but tight, opting for yoga, swimming and flexibility workouts makes more sense. On the contrary, the runner who is flexible but lacks strength should opt for weight training/strength training along with cycling or swimming. Ideally you should have a moderate level of flexibility and a good amount of core and hip strength to be the runner you have always wanted to be!