The aim of this research project was to conduct a manual development study and an open clinical trial in order to demonstrate the feasibility and efficacy of a mindfulness and acceptance based intervention for increasing cardiorespiratory fitness (CRF) in sedentary adults through adherence to a fitness walking program. Development of the treatment manual followed a 3-phase process (literature review and initial draft preparation, expert review, draft revision) based on expert systems analysis, and organizational structure was derived from Carroll and Nuro's Stage Model for Psychotherapy Manual Development. Field experts (N=3) were provided with the manual draft, as well as a semi-structured interview form for revision data. The manual included treatment introduction sections for the therapist and the participant, as well as 8 topic modules. In the10-week open trial, sedentary adults (N=24) engaged in a fitness walking program, while attending regular group therapy sessions whose content was based primarily on Acceptance and Commitment Therapy (ACT). Results indicated a large significant decrease in total walk test time [t(18) = 4.61, p = .0002, d = 0.64], with a mean decrease of 64.69 seconds. A moderate significant increase in estimated V0₂max [t(18) = -4.05, p = .0007, d = -0.43] was also evidenced, with a mean increase of 2.9 ml/kg/min. Analyses indicate a moderate non-significant increase in general experiential acceptance as measured by the AAQ-II [t(18) = 1.18, p = .26, d = 0.37], and a large significant increase in experiential acceptance of exercise-related internal experiences [t(18) = -9.19, p < .0001, d = -2.09] as measured by the PA-AAQ. Finally, feasibility and acceptability of the intervention were demonstrated through high levels of adherence to the walking program, group attendance, and measures of comprehension. This study demonstrated the usefulness of ACT in the field of behavioral medicine, particularly with health behavior change.