I set ten miles as a max distance; like I said, you're out running errands/at the store/whatever and find yourself in a situation where you have to walk home, so what basics would you like to have along?
In the event of some type of disaster, might take quite a while longer to walk in than in normal conditions, so some water would be good. Spring/summer you could have warm to hot weather(water good) with a chance of rain. So I've got two bottles, and a poncho. I've almost always got a hat with me or in the truck, if I didn't I'd throw one in; sun out of my eyes and rain off my glasses.
I've got a MRE for food, or just some jerky or trail mix or whatever would probably do; something to munch on if the walk takes a while, or weather or other conditions cause you to decide to find a spot to hole up for a while.
Tam brought up 550 cord, a lighter and Charmin, all good thoughts. Strong cord can come in useful for all kinds of things, some way of starting a fire(just in case), and for either needing to blow your nose or wipe the backside the tp would be very nice. I think I've got a handful of those little packs they put in MREs, I need to throw a couple in. There's a box of kleenex in the truck, if had to could stuff a handful in.
A knife? You don't carry one? If you don't, or work won't allow or something, put one in.
A while back that horrible place Sportsman's Guide(damn them!) had some British web gear on sale and I picked up a set: shoulder straps and web belt, two magazine pouches in front, two cargo pouches in back. The ammo pouches hold water bottles, the cargo pouches the rest(and the stuff I'll add in after this). Bad part: it stands out more than a backpack would. Good part: better balanced than a pack.