The investigators propose a randomized controlled trial based on the Social Networks for Activity (SNAP) Model to evaluate: (1) the effectiveness of an internet-based fitness walking program delivered to patients with risk factors for cardiovascular disease and stroke; and (2) the contribution of social support networks to enhancing the effectiveness of the internet-based fitness walking program. The investigators will recruit 308 patients who are sedentary and have risk factors for cardiovascular disease and randomly assign them to one of the following groups: 1. Internet-based fitness walking program (WalkLink); 2. Internet-based fitness walking program plus additional social network intervention (WalkLink+); and 3. Minimal treatment control.

The primary specific aims of this study are to:

1. Evaluate the effects of WalkLink and WalkLink+ relative to a minimal treatment control group on change in: physical activity, aerobic fitness, blood pressure, body composition, and body mass index (BMI), from baseline to posttest, and at 6-month follow-up.
2. Compare the differential effectiveness of WalkLink+ to WalkLink on change in: physical activity, aerobic fitness, blood pressure, body composition and BMI from baseline to posttest, and at 6-month follow-up.
3. Evaluate the mediator and moderator effects of selected individual-level self-management skills (goal setting, planning, self-monitoring), social support variables (number of activity partners, social cues for walking, social praise/reinforcement), and physical-ecological variables (neighborhood walkability, physical cues for walking) on WalkLink and WalkLink+ treatment outcomes.

Inclusion Criteria:

• Sedentary, defined as less than 150 minutes per week of moderate intensity activity
• Between the ages of 35 and 64
• Able to speak English
• Able to provide informed consent
• Able to participate in moderate-intensity physical activity (e.g., moderate pace walking)

Exclusion Criteria:

• No access to home or private work computer with Internet access
• Body mass index (BMI) greater than 39.9
• Systolic blood pressure > 160 mm Hg or diastolic blood pressure > 100 mm Hg
• Bone, joint, or foot problems that interfere with walking
• Diabetes, pulmonary, or cardiovascular disease
• Consume 5 or more drinks of alcohol a day
• Currently pregnant
• Not living within the preselected study neighborhoods or plan to move from the local area during the study period