Siobhain, the basic need for aerobic fitness is this rule:
At least three times a week, not more than three days apart, for at least 15-20 minutes at a time.
So if you are doing your 20 minute shopping walk without stopping, three times a week with no more than 3 days between walks, you are in there!
The next thing is to try to add little extra exercise points all across your day. Park further out when you do errands, taker stairs instead of escalators, even for a few floors if not all of them.

House cleaning, if done in chunks of at least 15 minutes absolutely counts. Vacuuming is a great workout, for example. So is gardening. Or washing all your mirrors and windows - what a great arm workout! Or sweeping and mopping and counter wiping, etc.
Also you can add in physical play with your kids - dog walks, hikes, play frisbee, go swimming...kids are the BEST excuse to get physical activity in your day - you are multi-tasking. And it's better for both them and you to be doing active play whenever possible. Go climb trees! Do a yoga DVD together or a tai chi class.
I know we have a ton of posts about this kind of thing in various places but this is the gist of it.
