The distance you can drive in 8 hours is determined by your driving speed and how often you stop to take a break in those 8 hours.
For example if you were to drive at 65 MPH and drove straight through without stopping then you could drive 520 miles in those 8 hours.
But you also need to factor in stops for pee breaks, gasoline, stretching etc.
So even if you drove at 65 MPH you would likely drive only 400 to 450 miles in those 8 hours.
For example for myself when I drive 500 miles it takes me 10 hours to drive that 500 miles when I factor in the stops for getting gasoline, taking a break, eating etc.