This happened at a Corporate Challenge run I participated in many years ago. A guy in his forties, who was an avid runner, died in mid-stride near the end of a 5k race. Just went down face first and never moved. Probably had a congenital heart defect.
Enlarged aorta can burst and produce that outcome.
I knew a guy who died like that at 32. He was very fit, ran several miles a day, and died instantly on his morning run one day. I heard it was a brain aneurysm.
I am right there running a foot or two from the curb. As I get close, I see people rolling a guy over, off of his face and up onto the grass strip between the curb and the sidewalk. I recall that he was overweight, he had blood on his lips and chin, and his skin color was ash. All of the runners were getting overheated at that point and were a rosy pink to cherry red - but not this guy. I made the mental note - "he must be in his 60's" as I ran past, satisfied that there were more than enough people already going into action to help this guy.
Next day in the paper I see a headline - 33-year-old Runner has Heart Attack, Collapses During Race". An updated version of the story indicated he was DOA at Piedmont Hospital. I will never forget the disparity between my impression of how old he was based on appearance. and his chronological age revealed by his D/L.