@dpshoemanI remember reading a story with bits in it as you describe.
I cannot remember how it started but there was a sudden massive decrease in the world's population. The main thing I remember was that it is one of the very few stories about humans dying off en mass and the people left alive have to take precautions against animals, especially dogs. The MC gathers people around him and sets up a settlement with a defensive position. He raids army and air force depots for arms to defend themselves. They are attacked and a fire fight ensues which they manage to win. Other things happen, but this section ends with him going for ride on his horse and I believe he goes into a cave, regardless of where it was he gets a fatal bite from a snake. He realises it is the end and sits and calmly meets his demise.
The story then moves to a lecture hall of some sort where a lady, IIRC was a descendant of his, was telling the story of his exploits and how that restarted civilisation and everybody should be grateful for his work, all those years ago. Again, I am sure she tells about the things that are happening in her lifetime and that includes space travel.
Sorry for rambling on and CRS stops me from remembering the story name or author. If it is different from that dpshoeman requested and anyone remembers this one, please let us know what it was.