True SHTF scenario, the number of deaths would be staggering. Hordes would be even more at risk for several of the threats
What time of year does this occur? Winter and Summer both have risks associated with them - Winter probably being the worst. Not just deaths from cold, but a lack of anything growing, animals being laid up, roads and paths being more treacherous, etc.
Lack of medical care means minor injuries can kill. Minor illnesses can morph into deadly diseases.
Lack of abundant, clean water. Boiling doesn't fix everything
A Horde requires vast resources to maintain. There's a reason professionals study logistics. How much fuel, food, water does it take to keep a large group at fighting strength, and where are you going to find those resources as you get farther from cities? How are you transporting the resources needed to simply move a Horde, and how do you know there are resources to sustain/restock it where you are going?
The US is geographically huge, with large swaths being sparsely populated, and other parts only compounding all of the problems mentioned above (how do you get a Horde through the terrain you would need to traverse to get to SW Texas, for example? Better yet, why would a Horde feel the need to head to SW Texas?)
Hollywood glosses over that huge factor. Why would a Horde be where you are? Are you right on a highway that leads someplace that Horde might want to go? If not, by the time a Horde might get to you, lack of resources and die off will have taken it far below Horde-level.