I love walking to do my errands. Today I walked about four miles and was able to hit the bank, get my hair cut, buy some groceries, pick up a few things at Target, and check the dollar store for a couple of things (which they didn't have, it's kind of hit or miss there). This is a great way to get some exercise while checking off things on my "to do" list and I save gas this way too (not sure how much gas it saves, probably not much, but it does reduce my carbon footprint a bit).
Of course there are places where the nearest store can be miles away or there are days when you need a couple cases of water (or you need to do a Costco run!) which would make walking to do your errands kind of impossible, but for many people in urban areas or near the center of small towns, walking make a lot of sense.
Walking to do your errands also has the benefit of limiting what you buy. I bring a couple of strong nylon shopping bags with me to carry my haul but over-shopping isn't an option when I have to schlep everything back home.
Overall, walking to do errands is a win-win-win-win...exercise, save gas, get things done, and limit spending...I should do this more often!