You might want to consider checking out "voice actor schools " where people learn to do commercials for radio and TV. If you think about it, the typical radio ad spot is 30 to 60 seconds long, and even if you do 15 cuts until the producer likes one, you are not doing hours of continual talking. Audio books are a special segment. Being able to read well, and push out your voice are learned skills. Being adaptable is essential.
In the past, I did voice over narration for the University of Toronto's Medical School training videos for the nursing school. My Wife was a business manager in the Medical school so I was asked to do a 12 minute demo video about "how to lift patients safely ". I did it in 2 takes. That led to a lot more work, for various U of T departments over a period of ten years. It wasn't steady, but I set a price per hour that was below the usual amount charged by others, and I set myself up as a Registered Business entity in Ontario, and registered with Revenue Canada for taxation returns. I had my own equipment, and a sound editing computer program with a professional microphone and head set. JImB.