Despite awareness of the benefits of engaging in regular physical activity, at least 50% of adults in the US do not meet recommended guidelines for physical activity. One potential explanation for this lack of regular physical activity is that people often experience exercise as affectively unpleasant. Evidence suggests that the more positively people experience exercise (i.e., the better they feel while exercising), the more likely they are to engage in regular physical activity. This may be especially true for people in poor cardiorespiratory condition. In this randomized trial, investigators compared the effects of an affect-guided exercise prescription (intervention) to a heart rate-guided exercise prescription (control) on change in physical activity minutes among previously underactive adults. Investigators also tested whether the effect of the intervention was moderated by differences in cardiorespiratory fitness.