Speed

In my case, it's not the extra mile I'm pushing for but the thirty seconds that I tack on to my run during Friday's workout.  After walking for five minutes, I jogged fifteen continous minutes.  The question for the day--at what speed does a jog become a run?  Right now I'm jogging at 5 mph.  My  goal before transitioning to run outside April 1st is to last thirty minutes on the the treadmill.  So far so good but I will need to increase time by minutes instead of seconds if I am to reach my aim.