The P90X3 calendar suggested a rest day or the Dynamix workout. Since Sunday is my "long" run day, I opted for the run followed by Dynamix. Dynamix is one of the workouts in the "Core, Flexibility & Balance" group. The Beachbody website says Dynamix will "increase your range of motion, flexibility, and stabilization to help maximize the results you get from every routine".
Unlike P90X Stretch, which was the option on the rest day during the original P90X, Dynamix focuses on dynamic stretching. Dynamic stretching is basically controlled movements that help you increase your range of motion.
After completing this workout, I think it is something I will do before my run next week.
Longer than Last Sunday Run by tpengilly at Garmin Connect - Details