The World Health Organization highly recommends physical activities to control diabetes and obesity. Research on pervasive fitness devices is on the rise, but little attention has been given to how diabetic and obese patients use them to change their lifestyle. This paper reports the results of a one-month in-situ study with obese and diabetic patients. We interviewed 18 subjects using a set of carefully designed qualitative questions and let them use a fitness sensor called Fitbit for one month. Before the study, participants found it challenging to motivate themselves for physical exercise. One month later, they became aware of their sedentary lifestyles and started to perform more activities at a steady pace. Subsequently, patients reported rethinking of fitness and visible behavior changes.