Colorectal or peritoneal cancer patients scheduled for surgical resection (n=60) will be randomized to either the sedentary behavior intervention or monitoring only. The intervention will use a Bluetooth-enabled activity monitor to detect prolonged sedentary bouts, which will prompt a message delivered via smartphone suggesting that patients walk. The intervention will begin at least two weeks prior to scheduled surgery and will continue through the first 30 days at home following hospital discharge. Outcomes will include objective activity and sedentary behavior, patient-reported symptoms and quality of life using standardized instruments, inflammatory biomarkers, and morbidity and hospital readmission 30-days after index discharge.