Dalfampridine Combined With Physical Therapy for Mobility Impairment in People With Multiple Sclerosis

The goal of this clinical trial is to evaluate if combining a medication that can help improve walking in people with multiple sclerosis (MS) with a physical therapy program is better for improving walking than either treatment alone. The main questions this study will answer are: * Does combining dalfampridine with physical therapy improve mobility more than physical therapy without concurrent dalfampridine? * Is the combined treatment associated with better outcomes than the medication (dalfampridine) on its own? * How do the individual treatments (dalfampridine, physical therapy) alone compare to each other? Participants with MS-related mobility deficits will: * Receive 6 weeks of dalfampridine treatment to assess the effects of this treatment. * After stopping the medication for 2 weeks, the investigators will re-evaluate walking, then randomly assign individuals to a 6-week physical therapy program. * Half of the participants will receive physical therapy while resuming dalfampridine treatment. The other half of the participants will receive physical therapy without resuming the medication. Researchers will compare the combination treatment group (medication plus physical therapy) to the physical therapy only group to see if the combined treatment improves walking-related function. Approximately 3 months after finishing the physical therapy program, participants will undergo a final evaluation to see if the treatment effects have been maintained.