The goal of this clinical trial is to evaluate GetUp\&Go, a program for promoting increased physical activity in individuals at least 6 months post moderate-to-severe traumatic brain injury. GetUp\&Go is a remotely delivered 10-week program that includes one-on-one sessions with a therapist and a mobile health application (RehaBot). The main question is whether participants in the 10-week GetUp\&Go program increase their physical activity, and exhibit associated benefits in mental and physical health, relative to those who are put on a waitlist. * Question 1: Do participants who receive immediate treatment with GetUp\&Go show more increased physical activity, measured by accelerometer activity counts per day, and improve more on secondary outcomes, such as self-reported physical activity, emotional function, fatigue, sleep, pain, and health-related quality of life, compared to their baseline, relative to those who are put on a waitlist? * Question 2: Do participants who have continued access to the mobile health component of the intervention, RehaBot, show better maintenance of physical activity gains compared to those who no longer have access to RehaBot? * Question 3: Are individual participant characteristics associated with participants' response to the treatment program?