I have run 5K and 10K races on both sides of the island. To add onto satch74's suggestion, here is a list of upcoming races on Tri-Sport's web site: Tri-Sport Event Page. (Click on the Year's Planning tab.) For some races on the French side, a doctor's note certifying that you are in good condition is required.

My wife and I live in St. Maarten during the winter and in the Boston area the rest of the year.