Time is stored as a fraction of a day i.e. 1 = 24 hours, so to convert a time field to minutes multiply the cell contents by 24 * 60 = 1440
thus if A1 contains the time (in time format) and B1 contains the distance in miles then the number of minutes per mile is: (A1 * 1440) / B1
Remember when you enter that into a cell to precede it with an equals sign '=' to make it a function rather than just text. If you subsequently click on that cell to select it then you can click and drag the little square at the bottom right of the selected cell down the column to set the appropriate function in all those cells too.