I want to calculate average mile times for my runners

I coach XC and want to divide my runners' times by the distance they run to get an "average mile time." I enter my times in column C and put =C3/1.3 in column D, but I get this:

Error
Function DIVIDE parameter 1 expects number values. But '9:07.7' is a text and cannot be coerced to a number.

I've tried changing the format in column C to all different time formats, but nothing works. I have done this with Excel in the past, but Sheets doesn't seem to make this easy.