I would suggest that you post this question in the MS-Access forum. They have more experience with the Access GUI and might be able to give you better suggestions.
I tried using Cdate("13:23:45") in an Access query, and it worked nicely for me. I suspect that that is the conversion that you need, but I'm not certain about what code you need to derive a properly formatted time string.
There is no need to change the data type of the table. I just created a query with all the fields of the table but for Duration i put: CDate([Duration]). The query returns the results as Date/Time and from there i can perform the calculations i require.