Participants assigned to RPE-Rx will receive instructions to "select a pace that would make it challenging for you to carry on more than a short conversation." The exercise intensity prescription section of their exercise plan will include the Rating of Perceived Exertion (RPE) scale as an attachment and participants will be given the instruction to "stay in the green zone" on the RPE Scale.

Core intervention content will be delivered during the Visit 1, 1-month, and 2-month-check-in meetings. Definitions of exercise will be provided, participants will be asked to discuss their past experiences with exercise, guidelines for being safe during exercise will be reviewed, and strategies for overcoming potential barriers to exercise will be discussed. All participants will be given an exercise plan organized according to the FIIT Principle (Frequency, Intensity, Time, and Type). The intensity component of the exercise plan will differ by study arm, all other elements will be delivered the same for all participants. All participants will be given the goal of increasing weekly time spent exercising to ≥150 minutes, consistent with recommended guidelines. All participants will be given a smartwatch to support self-monitoring of exercise and activity levels throughout the 12-week study period.

Core intervention content will be delivered during check-in meetings scheduled during study weeks 7, 8, 9, and 10. During these visits, the interventionist will work with the participant to set weekly goals and problem-solving barriers to action. All participants will continue to receive instructions to follow their assigned exercise prescription (Affect-based Exercise Prescription or Effort-based exercise prescription). The step-up intervention calls are delivered the same regardless to which exercise prescription format the participant is assigned.

The research grade ActiGraph wGT3X-BT model is a valid measure of physical activity intensity and duration among multiple populations, including breast cancer survivors. Minutes of moderate-vigorous physical activity will be collected from the ActiGraph wGT3X-BT every day for 10-days. Average minutes of moderate-vigorous physical activity completed per day of valid wear time will be calculated.

The research grade ActiGraph wGT3X-BT model is a valid measure of physical activity intensity and duration among multiple populations, including breast cancer survivors. Minutes of moderate-vigorous physical activity will be collected from the ActiGraph wGT3X-BT every day for 10-days. Average minutes of moderate-vigorous physical activity completed per day of valid wear time will be calculated.

An 8-item self-report questionnaire used to assess participant satisfaction with study participation using a Likert scale ranging from 1 to 4 . Scores are summed and then averaged; total scores range from 1- 4. Higher scores indicate greater satisfaction.

A 4-item self-report questionnaire used to measure intervention acceptability using a Likert scale ranging from 0 (not at all) to 4 (very much). Total scores range from 0 - 16. Higher scores indicate greater acceptability.

A 6-item self-report measure of exercise performed over the last month and week. Two items range from 1 (never) to 7 (often) and four items range from 0 (0 days per week) to 7 (7 days per week). Total scores range from 2-21. Higher scores reflect higher levels of voluntary exercise behavior.

A single item self-report measure that present six statements describing various levels of monthly physical activity. Response options range from 1 (I did not do much physical activity) to 6 (I did vigorous activities almost daily). Total scores range from 1-6. Higher scores reflect more vigorous, more frequent physical activities completed in the past month.

A 7-item self-report measure calculating the amount of health-related vigorous and moderate-intensity physical activity as well as sitting time performed by adult participants over a one-week period. The number of minutes per day participants report for 3 categories vigorous, moderate, and walking activities are multiplied by the average metabolic equivalent (METs) for each category (8.0 METs, 4.9 METs, and 3.3 METs, respectively). Scores for each category are summed to calculate a physical activity score for the past week quantified as total MET values. Higher METS demonstrate a higher total volume of physical activity completed in the prior week. MET values between 0-599 are considered "low active," MET values between 600-1499 are considered "moderately active," and MET values 1500 or higher are considered "High active." This measure also assesses participants' total sitting time in the past week. Total number of minutes reported for sitting time is not part of the physical activity

A 10-item self-report measure with response options ranging from 1 (poor/never/not at all) to 5 (excellent/completely/always) for items 1-9 and 0 (no pain) to 10 (worst pain imaginable) for the item 10. Total scores range from 9 - 45. Higher scores represent better health-related quality of life.

An 8-item self-report measure with response options ranging from 1 (unable to do) to 5 (without any difficulty) and the degree to which health limits specific activities on a scale from 1 (cannot do) to 5 (not at all). Total scores range from 8 - 40. Higher scores reflect better physical function.

An 8-item self-report measure with response options ranging from 1 (never) to 5 (always). Total scores range from 8 - 40. Higher scores reflect higher levels of anxiety.

An 8-item self-report measure with response options ranging from 1 (never) to 5 (always). Total scores range from 8 - 40. Higher scores reflect higher levels of depressive symptoms.

An 8-item self-report measure with response options indicating the level of sleep disturbance a participant may experience. The first question asks about sleep quality and ranges from 1 (very good) to 5 (very poor). Questions 2-8 further inquire about the specifics of the sleep experience, ranging from 1 (not at all) to 5 (very much). Total scores range from 8 - 40. Higher scores reflect a greater degree of sleep disturbance or poorer quality of sleep.

A 5-item self-report measure of intentions to exercise in the next week. Response options range from 1 (not at all likely) to 7 (very likely). Responses are summed and then averaged; total scores range from 1-7. Higher scores represent stronger intentions to exercise in the next week.

A 4-item self-report measure with response options ranging from 1 (Strongly disagree) to 5 (Strongly agree). Total scores range from 4 - 20. Higher scores reflect higher levels of pleasure when performing physical activity.