When the plantar fascia, the fibrous band that runs from your heel bone to the base of your toes, is placed under too much stress, it stretches too far and tears. This causes inflammation of the fascia as well as the tissues that surround it, making every footstep painful. Runners are susceptible to this condition since running can create tight calves, which pull on the fascia, increasing the tension on the fascia, decreasing its flexibility and leading to tears.
Aside from stretching the calves, keeping your feet flexible will help too. Simply stretching the sole of your foot by pulling your toes back will help to lengthen the fascia. Massing your feet with a tennis ball will also help, but I like to get a little deeper with a Rubz ball. Strengthening your feet with toe scrunches is also helpful. Do these daily, whether it's a running day or not.