Hi Scott,
Do you have flat feet? It could be plantar fasciitis, however i would avoid going to the doctor and go straight to a Sports Podiatrist or a Physiotherapist.
Make sure you are applying ice to the injuried area, or better still plunge your foot into a bucket of ice water for 10 minutes, several times a day, as whatever it is the ice will only help.
Have you changed anything with regards to your training, eg hills, distance, shoes?
Cheers
Nicole