Hi I am assuming that by installed activity you mean the "Run" activity as there are around 16 different ones to choose from. Having selected Run and allowed the GPS to connect simply press the right hand button to start or finish the run. You only use "Workouts" if you want to use any workouts you have created or the training calendar.
Hmm, strange that intervals are set on your watch 'by default'. Mine wasn't. Maybe you got a returned item. But, to turn off running intervals, from the running activity, select Settings - Alerts. You can either disable it with the slider on the right of it or select that Run/Walk and select Remove.
