Personal Informatics (PI) systems common trending theme is fitness, which in turn is a trending theme in human-computer interaction. These systems have been the focus of a handful of studies. So far, the research that has been conducted merely focuses on the technical details of such services and few investigate the longitudinal use of PI systems. Even though fitness is a popular area, still personal informatics systems designed for this purpose suffer from short-term use. Therefore, there is a need to design systems that encourages users for prolong use. The prolonged use of digital artifacts is an issue of sustainable interaction design. The purpose of this thesis is to explore the design space of personal informatics fitness systems through the lens of sustainable interaction design. This thesis aims at investigating motives and intentions behind the longitudinal use of personal informatics fitness systems. This thesis argues that by adopting the lens of sustainable interaction design we can find ways of designing PI systems with prolonged use for the users. Based on the findings the thesis ends by suggesting a guideline for designing personal informatics fitness systems which will hopefully help interaction designers and researchers alike.