Prospective Memory (PM), which involves remembering to perform intended actions, is the primary source of everyday memory lapses. While existing solutions mostly focus on supportive memory aids and reminders, it is also crucial to maintain PM functions and independent living for older adults. We present ProspecFit, which digitises implementation intentions, a lab-based memory intervention, making it available on smartphones through iterative design that draws insights from a focus group and preliminary studies. We evaluated its usability and effectiveness in enhancing PM through user studies that included a 12-day in situ study, and pre- and post-testing with 10 adults (61 to 80 years old). Participants in the digital PM training group were more prompt in performing the in situ PM tasks, compared to the control group without digital training, and reported improvement in their PM compared to before the training. We also show findings from diary entries, reports on forgetful moments and user reactions. Our work provides implications for creating digital memory training tools in HCI.