COS-DEN. If the runway directions align you are in the air 15 minutes.
I also did LAX-LGB once long ago on Air Cal, I don't remember how long it was but it was very short.
I'm trying to find route maps. As a bored kid before the internet, I'd just stare at them since they were free.
This one is supposedly from 1979:
Or even earlier in 1976: