Sorry about vague details but I am looking for a story about (?)humans living in a valley where they only live for four days- they are born and have one day as a baby, then a child, one as an adult then one as an elderly person- it's possibly post-apocalyptic. There is a means to escape this but it involves a dangerous journey across mountains and needs to be done in darkness as when the sun comes up they would be killed by heat if caught out in the sun. A couple manage to find a way to either escape or to find a way to save everyone. Can't remember any more. Thanks for any help!