I'd love to live in a place where everything was within walking distance, but I live in the suburbs.
The closest grocery store to me is about 4 miles, but completely sidewalked. My school is about 3 miles away, but there's no sidewalk and requires walking along one of the worst roads in my area (windy road with multiple housing developments attached to it). If I want to catch a bus, the nearest stop, I'm pretty sure the nearest stop is at my grocery store.
Like the article said - much of the country doesn't have the infrastructure, or planning, to really rely on walking. If I want fast food, or need something from the store, that's over an hour of walking each way - often in the heat or rain. However, doing any of those in a car would take maybe 20 minutes total. If I had to walk for everything, most simple tasks would become all day things. I couldn't even get a job, because the nearest business is 20-30 minutes away, and up a huge hill.
Bicycles aren't preferable, either. I live in a valley, so any place I want to go requires riding up a decently steep hill for about a mile. Then, I have to decide to either put others at risk by riding down the sidewalk (the number of times I've come across people walking and not paying attention to anything around them is high), or put my life at risk and hold up traffic by riding on the congested neighborhood roads that go everywhere. Still, the store is half an hour away, and many places are still dangerous to even think about going to. On top of that, the abundance of hills in my area are going to make me arrive hot and sweaty.
I might be a special case, but there's many Americans that are in the same situation. For us to rely on our own biological transportation, even with the assistance of mass transit, is still time consuming, and often dangerous. The bad part is - it's not going to change for us. Like the article said, it's hard to build walking paths after roads have been put down and buildings and infrastructure have been built around them. On top of that, the paving of the paths would be expensive, and the cost would rise in places where the terrain is difficult to build (There's roads around here that would require a lot of work to have a surface next to them viable for a sidewalk). The expansion of public transportation would be very costly, and there probably wouldn't be that much of a demand for it in places where it didn't exist before. People have already bought cars to get around, and aren't going to give them up in favor of a bus that they have to wait for and share with people.