The first question I would ask you is about your pain, as in my experience MS pain can often be treated medically and many of my MS patients have found that once they have achieved adequate medical management ofd their pain they were able to engage in exercise programs with much greater frequency and intensity.
It has also been my experience that much of the pain that persons with MS experience is not neuropathic but orthopedic in nature and therefore amenable to treatment with physical therapy.
On days when you feel like your fatigue rather than pain is keeping you from exercising, you should be aware that medications for fatigue are also availlable. My patients have reported improved ability to exercise after using medications such as ampyra and provigil.
Even on days where your pain and fatigue are preventing you from your regular exercise program, there are alternative exercise approaches which are less physically stressful but still can maintain or improve your fitness. These include stretching programs, particularly of your legs, aquatic programs, and breathing and meditation.
Persons with MS should always try to guard against immobility as it can seriously complicate many of the other symptoms. Therefore whenever pain, fatigue, or some other MS symptom prevents you from engaging in your regular exercise or mobility activity, you should speak yo your physician to see if there is a medical intervention, or a PT with experience in MS to investigate whether there is an orthopedic cause.