Specific Aim 2: The second specific aim is to examine the effects of 3-months of treadmill walking exercise training compared with an active control condition on hippocampal volume, hippocampal resting-state functional connectivity, and cardiorespiratory fitness in those persons with MS. We hypothesize that those who are randomly assigned to the treadmill walking exercise condition will demonstrate increases in hippocampal volume and resting-state functional connectivity (i.e., adaptive increases) and improved cardiorespiratory fitness relative to those in the stretching-and-toning condition.