Did you use the "Resume Later" function from the pause menu of the Trail Run activity? If so, then every time you press the top-right button it will re-enter your Trail Run activity... until you end the activity by discarding or saving it.
But since your timer is showing 0:00:00, I'm guessing the activity was never started, thus isn't paused, and that simply taking philip's advice above will exit the activity. Then press the top-right button again to bring up the list of all your activities.
