Depends on the provision on pavements/sidewalks and the weather.
The US is often very hot or very cold. Plus sidewalks are not regular. The country is set up for motor vehicles.
I expect most people in the UK would be unlikely to walk 15 mins each way to the shops. But would go longer on a recreational walk
I voted that 30 mins is a short walk because I used to walk more than that to get to school in Cornwall or going to Tesco's or the pub when living in Reading. But here in Houston I think I've only ever walked to the grocery store once in almost 20 years, and it's just over a mile away. It's too hot and humid in the summer, there isn't a contiguous sidewalk to get there and crossing the road, even when there are pedestrian lights, is risky. No one walks, therefore drivers never expect there to be pedestrians and so they never really check for them at intersections.
My wife when she lived here in Houston 30 years ago would often walk the quarter mile from her home to a local coffee shop and drivers would stop all the time to check to see if she was ok, and ask if had her car broken down, or did she need a lift.
I will say that most evenings I see a lot of neighbours out walking for leisure, getting their steps in for the day. But they generally suit up in work out clothes, walk in the road and probably only get out for a max of 30 mins. They think I'm crazy taking 2 hours walks in the local park on the weekends.