The investigators will use a 3-group design in 180 older (65-85 years), obese (BMI=30-45 kg/m2), sedentary men and women, all of whom will undergo a 9-month Weight Loss (WL) intervention (6-mo intensive phase and 3-mo reduced contact phase), followed by a 9-month self-managed follow-up phase with minimal contact, to test our overall hypothesis that intervening on Sedentary Behavior (SitLess) will enhance long-term Weight Loss in this age group. The diet element of the intervention is identical across groups, but groups differ by activity intervention: 1) structured, moderate-intensity, aerobic exercise (EX) (WL+EX); 2) intervening on SB throughout the day (WL+SitLess); or 3) (WL+EX+SitLess).