Hi Kate,
Having suffered your complaint recently but not being an expert on such matters I can only tell you how I have overcome this problem and back to doing my usual mileage. Firstly rest from running is really the best option, I finaly gave in and quit for a month. I done a bit of work on an elliptical trainer to keep some fitness, but I would advise not using a stepper as they can stress the injury unless you can keep your foot dead flat. also try not to stretch until all pain has gone. But the best thing that has worked for me is putting sorbathane heel lifts in my shoes, this has worked wonders for me,I can't say it will do the same for you but it maybe worth a try
