Garmin Forerunner How Can I Do An Interval Workout With My Garmin Forerunner 405?

How can I do an interval workout with my Garmin Forerunner 405? - garmin forerunner

I bought a Garmin Forerunner 405 and I did interval training without the time interval. I mean, I would only start the stopwatch and the rest, if I back button, when I press the "Back" button. But I do not want my peace is my calculated average rate / total time. Suggestions?