TrainingPeaks does not yet support the export of Duration-based workouts to Zwift. Zwfit actually just activated support for Duration-based runs, and TP is just a bit behind. Since almost all 80/20 run workouts are Duration-based, they cannot be exported from TP at this time.
This is up to TrainingPeaks at this point. In my last conversation with my TP account rep, he reported that Zwift wanted to get out of running solutions, and therefore TP was hesitant to put in the dev time if Zwift was to reverse course.
The best thing we can do is to individually confirm with TrainingPeaks our desire for a Duration-based solution in Zwift.