We designed and implemented a just-in-time physical activity intervention system, WalkMore, which uses rich context extracted from smartphone and smart watch to encourage opportunistic walking without significantly altering users' daily schedules. This system incorporated several existing psychological theories such as Behavior Change Theory, Habit Formation Theory and Goal Setting Theory. We also conducted a 4-week user study to evaluate the acceptability and usability of WalkMore. In addition, the results of the study show that WalkMore is effective in encouraging people to walk more and engage other physical activities. Compared with reminders sent randomly in time, context-aware reminders are more acceptable to participants and more effective in increasing participants' self-awareness of possible walking opportunities.