Bad math.
There's 24*365 --> 8,760 hours in a year. If it were only that year it would be 51,000 / 8,760 --> 6 mph.
19 months would be 51,000 / 13,870 --> 4 mph.
Put a bit differently, 19 months is 578 days. 51,000 miles / 578 days --> 88 miles / day.
88 miles / day is high for a commute but...