Identifying useful features of health and fitness technologies has the potential to lead to more effective pervasive technology interventions. Through collaborative creative processes, co-design provides useful insight and direction for the design of these technologies from populations such as older adults who are often neglected in design. Additionally, extensive technology experience prior to co-design may elicit richer feedback from users that may otherwise be unfamiliar with the intervention of interest. In this paper, we examine the value of experience-based co-design among older adults. We report on seven design sessions conducted with a total of 25 older adults, following a 10-week study of fitness app effectiveness. Sessions were grouped into users and non-users of the assigned app following the study. We found that continued use with the assigned application led to more robust, detailed feedback in design sessions, suggesting that long-term, prior use of sample technologies is an important prerequisite to ideating useful features for new health technology. We draw on these findings to make design recommendations, and discuss the values and guidelines of implementing experience-based co-design.