It's just insane to think about how much
* physical effort
* time
* laundry
* food
the typical sub-sub-elite runner puts in to making minor improvement like 15 seconds over 5 kilometers.
Most of the time they never improve because they get hit with injury, age or something else that can wipe out their speed.
Think for a minute about how stupid this is: we train 70+ miles per week to run 3.1 miles. So unproductive.
Compare to another hobby like chess. You could become very good at chess of you invested 10 hours per week. You don't have to worry about injury or any other external factor wrecking your skill (other than something rare like a brain stroke which would take you out of any hobby). It would cost nothing. And if you took a year off, you might only loose 3% of your skill, whereas in running you would lose 90% of your skill and it would take forever to get it back.