The rural areas are the majority of the land in the US, they aren't even close to the majority of the population. And no, most of them aren't sitting on weeks to months of food. They are also worse off in many cases in terms of travel than those in the urban and suburban areas because of how many rivers and streams the US has crisscrossing most rural areas.
As for water based transit, again you need to get things to and from the water and the waterway needs to be 1) able to handle barges and 2) actually have sufficient barges on it to move the goods in a timely manner.
This isn't just a major infrastructure collapse; this is about the worst infrastructure collapse imaginable. Bluntly speaking, it would probably be less damaging to simply have every single power plant just up and disappear one day. A wholesale nuclear war or the total destruction of every single city with a population over a hundred thousand would be less devastating that making all bridges just poof out of existence.
Building a bridge that can handle modern transport needs over even a relatively small stream is not a quick or easy process. Building one over a proper river? That is a year plus even as a high priority project. Sure, you can do emergency bridging but it won't last through any kind of weather and we don't have anywhere near the amount of bridging equipment that we would need to have a noticeable impact on the situation. And you can't run a train over anything but a proper bridge.
The entire highway network, especially around any kind of population center, is also shot. Again, no overpasses (they are all bridges). That alone makes it practically impossible to actually drive into or through any urban area in the US.
