ok, this may sound stupid, but you might want to consider adding this to your repertoire (seeing as you have an 18 month old...)
elmo.
yep, elmo makes two exercise videos for kids. one is called "elmocize" or something to that effect, and the other is "happy, healthy monsters". something like that would probably get you moving (albeit in a silly way), but also allow you to get it done with a child that probably doesn't have much of an attention span for watching adults dancing around. there are some other "made for kids" exercise videos, too. you could look into it, at least for the days that you will be exercising with your child. if your fitness level is really in the minus area, any type of movement would be good for you. and besides getting you to move, it would help your child grow up with healthy habits! i usually do my exercising at night, after my 16 month old daughter goes to bed, but we have both the elmo videos and she sometimes picks them out and we do them together.
gets some exercise in while giving me some good play time/interaction with my daughter. its actually my favorite form of exercise... watching that cute little diaper butt imitating elmo... hee hee!!!
good luck to you!!!