Often a transition from couch potato to active individual is brought about by an upheaval.  Again, a sufficient lever to promote you into action as opposed to sitting back and being 'reactionary'.  This can be a threat of ill health (such as a doctor's dressing down of your lifestyle), a failed attempt at a 20km hike or a sudden realisation that you are under and below a level of health/fitness that you thought you were.  Seizing on these 'opportunities' are a great way to propel you forward into your new habit forming action.  Seizing that moment and beginning 'now' rather than later creates urgency which results in you being more likely to follow the momentum you are creating.