You cannot prepay for mileage. Of course it may be possible to choose a supplier that offers UNLIMITED MILEAGE. If you look here, it will say:-
If the car only has LIMITED MILEAGE then click on the link next to the car photo ''IMPORTANT INFORMATION'', then choose the car TAB. Here it will say how much per mile / km will be charged and how many are included or free in the rate.
You would pay for any mileage you use at the end of the hire period.