Background: Sedentary behavior and physical inactivity are modifiable risk factors at the forefront of prevention and health promotion strategies. The health benefits of physical activity (PA) have been widely demonstrated in chronic diseases and have led to the prescription of adapted PA. To date, French scientific data are insufficient to evaluate the effectiveness of prescribing adapted PA. This study aimed (1) to evaluate the effectiveness of a community-based program and (2) to identify patient characteristics at inclusion that could be associated with improved post-program anthropometric data. Methods: Our sample was composed of 113 participants with a chronic disease (83.18% women) with a mean age of 55.4 ± 13.9 years. Participants benefited from an 8-week adapted PA program. All participants were evaluated at the beginning and end of the program by anthropometric measurements, a subjective measure of the level of PA and a measure of physical condition. Results: Almost 86% of the participants were overweight and two-thirds were obese. Statistical analyses showed a significant improvement in physical condition, expressed by a better cardiorespiratory endurance (up to +14% for a 2 min walk test; MT0 = 78.1 m vs. MT1 = 89 m; p < 0.001 with a 2 min walk test), improved flexibility (+12.5%; MT0 = 2.4 vs. MT1 = 2.7; p < 0.001), and increased muscle strength in the lower limbs (+22.7%; MT0 = 11.9 vs. MT1 = 14.6; p < 0.001). The level of physical activity increased significantly for all participants (57.52% of inactive individuals at T0 vs. 5.31% at T1; p = 0.004). Correlational analyses revealed that the decrease in BMI and weight throughout the program correlated positively with age (r = 0.252 and p = 0.007, and r = 0.247 and p = 0.008, respectively) and negatively with BMI from baseline (r = −0.271; p = 0.004). Conclusions: The key points of this community-based PA program are the following: (1) It improves participants’ physical condition. (2) It improves anthropometric parameters. (3) It modifies physical activity behavior. Furthermore, in the context of the program set up specifically for this purpose, it would appear that elderly and overweight people are more likely to exhibit beneficial effects on anthropometric parameters than younger participants or those with a high level of obesity. However, these results need to be confirmed by a long-term evaluation of the effectiveness of such devices.