It is much easier to work with times in Excel if you input them as Excel times e.g. hh:mm.
However, we can get around that given that you've entered the data in the way that you have (assuming that you only have whole numbers, no decimals).
In a spare column (say J) in J6 enter the formula
and fill the formula down as far as needed.
Then to get the total duration use the formula
and format the cell containing that formula as Custom [h]:mm