What is the field type of your duration/run time field?

If it is a duration field type, it is stored in seconds. You can adjust the formatting settings for the field type to show the mm:ss format.

If the field is a number field, you can use a formula field to divide by 1000, then set the formatting for the field to duration and set the mm:ss duration format.

If you need to format the number as a string to include in another formula, I’m afraid that is a bit more difficult, but still doable.

This was very helpful. Thank you. I had it stored originally as a single line text, but I just coverted it to a number field. Then, I was able to add a formula field, divide by 1000 and format properly. Works great. Solved. Thank you!