For the walking speed outcome, the evaluable sample size of 30 in each group will have 80% power to detect a difference in means of -0.206 (m/s), the difference between an assumed usual care walking speed of 0.51 (SD=0.28) and an assumed intervention group mean walking speed of 0.716. This assumes a common standard deviation in the two groups and a two group t-test with a 0.05 two-sided significance level. The estimates for the mean and standard deviation for walking speed come from the LEAPS RCT (Duncan, N Engl J Med, 2011). The investigators plan to enroll 66 patients to account for an expected 10% drop-out rate. The t-test used for the power calculation is a simplification of the mixed effects analysis plan for the primary endpoints.