Injured?

Went for a run this morning as part of my training to run the Bath Half marathon.

Unfortunately about 3.5 miles in, I appear to have done something to my lower calf/achilles tendon area. It's quite painful, though not when I'm not using it (which I guess must be a good thing). Hopefully it won't take too long to heal as I can't really afford to get too much more behind training.