There is a difference between supervised exercise training, physical therapy, and "lifestyle activity". For years, people with MS have been encouraged to seek physical therapy. But they have not been encouraged to simply move more. For those with MS who are still able to take steps and walk, I'd like to offer some information, encouragement and cheerleading.
The MS activity paradigm is changing. Here is a recent abstract on this topic:
Lifestyle physical activity in persons with multiple sclerosis: the new kid on the MS block.