That sucks you might have to miss a race because of an injury. I feel your pain; it's really frustrating.
I had really bad compression fractures in both shins. It was probably a combination of bad running shoes and poor running form. They were very painful, to the point I could not walk without limping. Only time and rest allowed them to heal up right; it was too painful to apply ice but that may have helped.
Nowadays I don't do any high impact cardio more than once or twice a week. I run on softer surfaces with better shoes and I've worked on my form as I run, and it has helped considerably. I also think working on calf strength has helped me. I do a lot of these:
http://www.exrx.net/WeightExercises/Gastrocnemius/BBStandingCalfRaise.html Basically, do what it takes to heal up your compression fractures, and let them take whatever time they need. With some training they'll come back stronger than they were before and you'll won't be missing the next 5K.