Background: Physical inactivity is a modifiable risk factor for dementia, but there remains a research translation gap in effective physical activity (PA) implementation, particularly in the longer-term. The INDIGO trial investigated the effectiveness of a six-month PA intervention for inactive older adults at risk of cognitive decline with subjective cognitive decline or mild cognitive impairment. Objective: This follow-up study aimed to collect feedback from INDIGO participants about their experience of involvement in the trial, including barriers and enablers to longer-term maintenance of PA. Methods: A qualitative study using semi-structured individual interviews was conducted and transcripts analyzed thematically. All INDIGO trial completers were invited, with 29 participating (follow-up period 27–66 months post-baseline). Results: At long-term follow-up, participants described INDIGO trial participation as beneficial. The theme of “Motivation” (subthemes: structure and accountability, knowledge and expected benefits, preferences and motivation, tools) followed by “Situation” (subthemes: environment and time, social aspects, aging and physical health) appeared to be critical to PA “Action". Most participants had a positive view of goal-setting and peer mentoring/support, but there was some polarization of opinion. Key factors to longer-term “Maintenance” of PA were self-efficacy and perceived benefits, habit formation, and for some participants, enjoyment. Conclusion: PA interventions for older adults at risk of cognitive decline should include behavior change techniques tailored to the individual. Effective techniques should focus on “Motivation” (particularly structure and accountability) and “Situation” factors relevant to individuals with the aim of developing self-efficacy, habit formation, and enjoyment to increase the likelihood of longer-term PA maintenance.