Does anyone know the name of the short story in which the characters only lived a few days from birth to death? I believe it involved a ship where time was experienced normally. I recall the story plot from my younger days, but don't remember the author or the title.
Sounds like "Frost and Fire" by Ray Bradbury.
Could it be Ray Bradbury's Frost and Fire (1946)? See this question.
Placed there by a past rocket ship that crashed, the people of the storied land are within sight of another rocket ship on a distant mountain plateau. The plot follows Sim, the protagonist of this story, and his apparently short life on a planet where people are cursed by radiation to live only eight days.
You most likely read this in R is for Rocket.
Once they're in the ship, they indeed do experience time normally:
He could not believe it.
Her first words were, "How long will this last?"
He replied, carefully, "I don't know."
"We are still young."
"The ship. Its metal is around us. It cuts away the sun and the things that came from the sun to age us."
Her eyes shifted thoughtfully. "Then, if we stay here—"
"We'll remain young."
And they save some of their fellow men:
The weeks passed. Generations lived and died in the cliffs, while the scientists and workers labored over the ship, learning its functions and its parts.
Not quite "only a few days", but part of the plot of Non-Stop by Brian Aldiss is that
the spaceship which the lead character and his tribe are living on is actually a generation ship which has returned to Earth orbit, with its inhabitants having developed to be smaller, move much faster, and die sooner. Their slower "Giant" opponents are actually normal humans.