Being a radio announcer is not a “1 day job” either. You need to do hours of research not only on your team but the visiting team so you are aware of roster, stats, and current storylines.
Generally, this involves putting together either a physical or electronic document you can quickly pull from. In Kesling’s case, he had a large trifold that he developed each week for games.
I would say being a skilled radio announcer is at a minimum a 3-4 day a week job. This is a big reason why our announcers generally do two sports at most concurrently.