|I enjoyed some of the descriptive language and prose in your paragraphs once you reach the straw-man. I will critique the rest bluntly but that is the way I feel we as writers can gain the most. No offense, and these are my artistic opinions and not fact.
I believe your opening paragraph, the characterization of Martha, and the way you introduced the myth should be rewritten entirely.
Since you are using first person narrative, it would be more suspenseful within your story, if you focus on the inward fear and uneasiness of the main character. The dinner conversation with Martha did not make me feel connected to them like they were just a regular couple and that nothing was going to happen. It did the opposite, I wanted to stop reading. Here is why it feels constrained, I think the whole purpose of that portion of the story was so you had a setting to explain the myth. Is the backstory at the start necessary? I.E If the main character encounters the straw man, as he runs away, he can think, I don't want to die like Simon, I thought it was a myth, I just got over the circumstances of his horrible death, now I am chased by the very thing I convinced myself did not exist... (of course you would frame it your own characterization of the narrator)
If you keep Martha in the story, you need to make her relevant, not just the main character being nervous and a little sad when she dies. She must serve a better purpose than that. In the current state, if she is removed, your story is improved. Her doubt of the myth come off as cheesy because right after she says take you to a psychiatrist, death happens.
That being said, you have a good foundation for a story. You have the ability to write descriptive prose and purposeful movement of the plot. I think you should put some more time into the issues I had while reading it, and decide how to address them in how you see fit.