You can and it is great! I have a bike and a treadmill and you can walk or ride and watch tv ot listen to music. Also, if you hit a slump try interval walking. It really helps. Walk regular for about 3 or 4 minutes then for a minute or 2 jog or walk really fast, then regular again, repeat it for however long you walk.
Those little burst sure do help when you hit a stand still. Or when you watch tv, exercise during the commercials (jumping jacks, jumping rope). I know it defeats the dvr, but it helps jump start the metabolism.
I know what you mean about the "woman-over-40" thing. My doc told me it just happens when you hit middle age. I told him, well I guess I will just have to fight it because I sure don't want to accept it. It is a fight for sure, but one you can win!