During baseline, participants will first complete a battery of neuropsychological tests, followed by an incremental exercise test to exhaustion on a motor-driven treadmill to measure cardiorespiratory fitness, and will undergo a MRI scan. Those outcomes will be measured by assessors who are blinded to condition. Participants will undertake several neuropsychological tests addressing various aspects of learning and memory. These tests include the California Verbal Learning Test-2 [Delis et al., 2000] and Selective Reminding Task [Buschke & Fuld, 1974] as measures of verbal learning and memory, and the Brief Visuospatial Memory Test-Revised [Benedict, 1997] and 10/36 Spatial Recall Test [Rao et al., 1984] as measures of visuospatial learning and memory. Cognitive processing speed will be measured using the Symbol Digit Modalities Test [Smith, 1982], verbal fluency will be measured using the Controlled Oral Word Association Test [Benton, 1994], and executive control (i.e., inhibitory control) will be measured using a modified flanker task [Eriksen & Eriksen, 1974]. Cardiorespiratory fitness will be measured as peak oxygen consumption (VO2peak), using an incremental exercise test to exhaustion on a motor-driven treadmill and an open-circuit spirometry system (ParvoMedics True One 2400, Sandy, UT) for analyzing expired gases using a modified Balke protocol. This protocol is commonly used for measuring cardiorespiratory fitness in older adults [e.g. Colcombe et al., 2004] and persons with chronic stroke [e.g., Macko et al., 1997] and further is consistent with the ACSM guidelines for exercise testing of MS patients [ACSM, 2013]. Lastly, participants will undergo structural and resting-state functional connectivity magnetic resonance images of the hippocampus. The MR instrument that will be used is an FDA-approved Siemens 3 T clinical imager. Following baseline testing, participants will be randomly assigned to either the exercise training or active control condition using concealment. Participants further will blinded to condition (i.e., unaware that the treadmill walking exercise training condition represents the experimental condition and the stretching-and-toning condition represents the active control condition).