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.