Do you have a pedometer? Many of them record how many miles you've walked, in addition to a number of steps. You could see how far you've traveled in a typical shopping trip, and how much time you spent doing it, and just divide the time by the number of miles (and you can just type the calculation into a google window, so you don't even need a calculator!). I find that if I'm walking at a constant pace, but not trying to walk as aerobically as possible, my pace is typically between 24 and 30 min/mile. (of course, I'm pretty short, so that might have something to do with it--my steps aren't nearly as long!)
Nice job fitting calorie burning into all parts of your life!