Hey,
The finger points not the shin but to the medial side of the knee joint which could end up being 'n' number of things. my advice would be to elaborate on the problem and till then start with mild icing on the pain and also as for a marathon runner be true to your warm up, stretching and cool downs so as to perform better at events. You can try doing heel and calf raise in sitting position if it doesn't increase the pain. Strengthen your lower limb muscles well so that you are able to perform stronger in your marathon. Looking forward to hearing from you and then helping you.
Thanks
Dr. Utkarsh Narang (PT)