Welcome Cynthia from another Orygunian! An overweight 60 year old with bad knees.
Just take small steps to start, and those "steps" will get larger MUCH more quickly than you imagine.
You don't want to hurt any body parts or get overly sore muscles, because that tends to make you want to park the bike instead of ride it. Limit yourself to a couple miles the first day and your body will tell you what you can do the next. Besides, you'll look forward for tomorrow if you didn't get enough the first day!
I don't have a car, so I do all my grocery shopping on a bike, using a back pack. It's a rare occasion that I wish I had a rack & panniers! I'm only shopping for one though and have a 2 mile RT. It sounds like you have a greater distance and more bodies to feed.
Something I do, is always have a grocery list made up. If I happen to be going by the store while on other errands, I'll drop in and pick up a few items that I can hang on the handlebars. I can stretch a couple of the store type plastic bags between my shifters, which is also where I carry things like eggs and other fragile items etc. (it used to be chips)
One thing I would suggest, at least for the dry weather, is to remove the fenders to save some weight. It makes hills and accelerating that much easier.
Use 1 lower gear than you think you need and SPIN. Mashing (pushing hard) is tougher on the knees and also WEARS YOU OUT faster! Spin, and you can spin all day (almost).
When going up hill, anticipate your down shifts a bit early and keep spinning. If you wait until it's too late, it's TOO LATE!
Keep your tire pressure at the max. It makes for easier rolling.
You might read time2start's blog. Very similar to your situation-
http://time2start.wordpress.com/