September 16, 2017. Duxbury, Masaachusetts. The Duxbury Beach Triathlon is loved for its unusual 1/3-mile straight-line swim across Duxbury Bay, a rolling 13-mile bike and a looping 3-mile run. This course will be one of the safest and most scenic triathlons you will do all year, with police detail officers and volunteers positioned at key points along the course. The event is expected to draw up to 800 participants including a Pro/Elite division, and many participants from area police departments will compete in the special “INLET” — International Law Enforcement Triathlon division.