Undertreatment means that “30 to 40 percent of people with simple ankle sprains develop chronic long-term joint pathology,” said one presenter, Tricia Hubbard, the undergraduate athletic training director at the University of North Carolina in Charlotte.

“Most research is showing that with any ankle sprain, the ankle should be immediately immobilized to protect the joint and allow the injured ligaments to heal,” Dr. Hubbard said in an interview. “At least a week for the simplest sprain, 10 to 14 days for a moderate sprain and four to six weeks for more severe sprains.”
http://www.nytimes.com/2009/07/28/he...20ankle&st=cse

I can imagine how badly you want to do this race and - if you're anything like me - how hard it is to rest an injury. But as the article states, it's just not worth the risk of long-term damage. Rest, heal, get good PT, there will be other races.