You already converted to hours in the duration column. You don’t need to multiply by 24 a second time in the average speed column.
To correct the duration column, I think you can still add Round to the begining of the formula so you don’t get a result formatted as a duration. The parentheses are perfect as is. The only question would be if you want to round the duration to a whole number, or if you want some decimals included. You can specify the number of decimal places you want like this to get 2 decimal places.
Round((end-start)*24, 2)