What should I do with the concerning the tendinitis in my foot?

I'm training for a marathon by running long distances, but there is a pain in my foot that won't go away after a recent training session. Every time I flex the tendon connected to my big toe I have this kind of rubbery feel, like when you rub two pieces of rubber against one another. I believe this is tendinitis. I know that I should probably stop running until its gone, but I've waited over a week now and my marathon is coming up very soon, so I don't want to get out of shape right before the marathon. Is there anything I could do that would allow me to continue running, while allowing whatever is afflicting my foot to heal as well? Perhaps a certain drug or supplement I could take that would speed along the process? The pain is centered around only one area of my foot. I believe the area is called the flexor hallucis brevis . Thanks in advance for any advice.