Unfortunately, failing legs are often a first sign of something going wrong, neurologically, and indeed, having legs that "feel like cement" are pretty common. Walking more, or exercising more, won't change the underlying condition -- the nerve impulses from your brain to your legs aren't getting through (the myelin is destroyed, or at least damaged, and the signals are having to re-route themselves). Walking more will only tire you more. It won't damage anything..but you will get more tired.
Obviously your doctor thinks you are impaired enough that he is ordering a motorized chair for you, so it's not like you're making it up, or baing a baby! I'm sorry your husband (or family) is being so unsupportive. They don't understand what MS is, apparently, and think that you're "out of shape, and if you just walk you'll get stronger." That WON'T happen. It's not that your muscles are "out of shape" because you don't exercise..they're "out of shape" because they're not getting the proper signals from the brain.
You'll just have to tell them they don't understand, and enjoy your new chair. You might investigate the power chairs that are available in many stores, too -- they're slow and cumbersome, but at least you'd be able to get around more easily and do your shopping.
You are describing the most difficult part of MS. That is the adjustment to a new you with different abilities. It does not matter what you were before this disease - this disease changes you and that is horrible. Please know that you do get past that adjustment period. I obssessed for a long time about all the new things I had to do to stay as "able" as I can be. I have changed everything. There are no more late nights. I have to eat extremely well with copious amount of fiber and water plus a few helpful supplements. I have to stretch and keep spastic muscles in line. I have to take meds to treat the progrssion and I have to take meds to treat symptoms. I ration my energy to get through each day. I still work but I spend each evening and weekend recharging my batteries. I have lost "normal". I resented this so much at first. Soon, it become ok to not have "normal" anymore. You learn to live with an everchanging normal - that becomes your normal.
Please know that as bad as you feel right now, we have all been there. It is really rough. I have just recently left that place you are in now. But things do get better. You will learn to appreciate your body again. You will marvel at the courage it takes to find joy in this life even if others don't see it. We know of your courage!!! So hang in there - your feelings are all normal and you are doing just fine.
Love and prayers,