The exercise training will include 3-months of supervised, progressive light, moderate, and vigorous intensity treadmill walking exercise, and will be based on ACSM guidelines for exercise prescription [ACSM, 2013]. We selected treadmill walking exercise as the aerobic exercise modality based on pilot work that suggests that treadmill walking might exert larger beneficial effects on cognition compared with stationary cycling [Sandroff et al., 2015], and that such benefits occur for light, moderate, and vigorous intensity treadmill walking exercise [Sandroff et al., 2016]. The exercise training itself will take place 3 times per week over 3 months, led by trained exercise leaders, and will initially consist of 15 minutes of light-to-moderate intensity treadmill walking exercise and eventually progress to 40 minutes of vigorous intensity exercise by month 3. The stretching-and-toning activities will be included as a minimal exercise, active control condition. The control condition will be delivered using the same frequency and duration of the treadmill walking exercise condition. The stretching-and-toning activities will be based on a manual provided by the National Multiple Sclerosis Society, and stretching-and-toning activities will be progressively included over the course of the 3-month study period. Participants will further be asked to not undertake additional exercise (i.e., not join a gym and begin exercising) over the duration of the study. The cognitive, cardiorespiratory fitness, and MRI outcomes will be assessed again following the 3-month study period by assessors who are blinded to condition.