I did it one year, 7300 miles but I worked nights. I did 30 mile rides before work twice per week. I did 100 mile rides 23 times that year. That would be on Saturdays, Sundays were followed by 40 milers. Other Sats that I did not do 100, I was doing 40-60 both weekend days. 40 miles takes me about 2.5 hours.
I did a solo 100 mile ride averaging 17.5 MPH on the average. I did one 100 miler with 10,000 ft of gain at 14.0 average speed.
So not like it was taking me anywhere near 8 hours per day to get in the mileage.
I had a great year then the next year I did 6,000.
The year after that, I felt like I was cycling and working so I tapered down to 4,000-5,000. Got to see my wife more once I got on dayshift.
Last 3 years have been plagued with family illnesses and deaths so hopefully 2020 I can get some better mileage going on.
I remember a thread with a guy stating he thought there was no way a person could ride 2,000 miles per year while working an 8 hour job. If there is a will, there is a way. My wife worked the last 22 years, 10 hour days and still managed 3,000 miles per year, several years of the last 22.