With the grocery store half a mile away, I would definitely just take frequent trips and buy a few things each time. You can also consider walking if you're buying bulky but not heavy items. I walk to my grocery store that's half a mile away year-round (granted, don't need to buy water and Seattle weather is much milder than most places).

Lastly, consider buying a water filter (Brita or PUR) so you don't need to buy water all the time?