Behavior change apps are widespread, but the scientific base of the app-concept is rarely disclosed. The aim of this article is to present the methodological approach used for the development of a so-called “fitness feedback demonstrator” within an already existing journey planner web app to motivate people to increase their physical activity behavior while using public transport. Firstly, we introduce the behavior change theories applied for the design of the feedback, followed by the analysis of focus-group discussions about the desired content of the fitness feedback. Secondly, we describe how we conducted a field test to measure the number of steps taken when using public transport. Finally, we used the feedback from potential users in terms of design/attractiveness and comprehensibility of the added fitness information. The “fitness feedback demonstrator” is a good practical example of how to make use of the different research expertise to develop a theory-based tool to encourage persons to integrate physical activity into their daily routines.