This study is a randomized trial that will compare the effects of two, 12-week activity interventions on patterns of physical activity, physical health, and quality of life measures in 40 older adults who are able to walk for exercise. One intervention will target 150 minutes per week of home-based moderate exercise (e.g. brisk walking), consistent with current recommendations. The other intervention will target a decrease in time spent in sedentary behaviors (e.g. sitting) of 60 minutes per day. Both interventions will wear an activity armband which will allow them to self-monitor their activity or sedentary behavior in real time using a smartphone. The armband will also provide objective data to an interventionist that will facilitate the intervention. The main outcome will be time spent in moderate exercise.