I run 15-20 miles/week and lately have had horrible inner calf pain that radiates from ankle to knee, what is this?

Many possibilities..: This could be many things and i would recommend seeking complete medical evaluation to get an accurate diagnosis. Common causes of these symptoms can be shin splints, medial tibial stress syndrome, periostitis, stress reaction or stress fracture, or exercise-induced compartment syndrome.

Overuse injury maybe: You may be suffering from the initial stages of an overuse injury. If you have pain along the tibial (shin) bone, you need to be examined by your trainer, therapist or physician. Try decreasing the mileage or intensity of your regimen for a week or two by limiting the impact aerobic exercise (running, jumping, etc.). Your response to this training change will help your examiner formulate a tx plan.