3. . The BA+GS condition (i.e., Group B) will receive the Base-App plus a fully guided goal-setting module (GS). This module is based upon health behavior theory, emphasizing gradual exercise progression and highly specific, realistic, and measurable goals. The module will guide participants in setting both long and short term goals: each week, those with access to the GS module will be prompted to revisit and reset their goals. At this time, participants will select several aerobic and non-aerobic activities for the week, and will set a frequency (days per week) and duration (minutes per session) for each type of activity. To emphasize gradual progression, participants will receive a minimum, maximum, and recommended frequency and duration, and these will be calculated automatically within the app based upon performance in previous weeks.