Create run workouts using times rather than distances

I’ve created a few running workouts for Zwift (by creating them as bike workouts then changing the sport type, distances etc.) and would like to be able to specify individual steps by duration as well as distance.

At the moment, it’s only possible to specify distance in the custom run .zwo files … even more confusing is that the distances are specified using a “duration” tag. Many running training workouts use duration instead of distance (etc. 3x 8 minutes at threshold pace).

I can’t imaging mixing the two in a single workout, so perhaps a tag at the top of the workout specifying whether “duration” is in distance (m) or time (s)?