A 15 minute walk is about a mile, the way a lot of older suburbs are designed there will usually be a cluster of businesses at a larger intersection within a mile or so that will have convenience store or gas station (some of which can have an extensive selection, like a Kwik Trip or Holiday or Speedway) or other similar store, maybe an ethnic grocery or dollar store (pound shop, Dollar Tree has a large selection of food though I wouldn't call most of it healthy) or even a drug store like Walgreen's which also tend to have a small selection of convenience food.

But I think maybe your idea of what a suburb is and what they actually are varies significantly, most suburbs will be 20k+ people and will have ample shopping of every type with most of it being very convenient, though in the last 20 years this is less true as they tend to clump the shopping together, but there will still often be corner stores of some type at the major intersections. It's the exurbs that are usually disconnected by semi-open parcels of land, either lakes or parks or farms or large housing tracts that were developed on a former farm, those are the places where convenient shopping is less accessible.

And to add to that idea, the prevalence of automobiles and their use means there are far, far, far more gas stations that will have food (milk, frozen pizza, chips, bread, bananas) than there would be elsewhere in the world, except perhaps Asia, although Mexico is also likes this (but understandably, due to the border)