•A nurse led primary care walking based physical activity (PA) intervention showed increased step counts at 3 and 12 months.•We explored the factors that promoted this long-term behaviour change.•We selected intervention group participants who increased PA and those who had not to explore facilitators and barriers to behaviour change.•We expected the two groups to give contrasting narratives but these did not emerge.•The groups differed in the identification of the same factor, eg social support,as either a facilitator or barrier.•Participants who sustained increased walking participated in the trial as couple and had a companion to walk with.•Those who did not improve lacked these, had higher levels of chronic illness and were more sceptical of the goal setting element of the intervention.