With the new update I now cannot find where to modify an existing custom workout on my ipad and its very frustrating. I can only start the workout, delete it, or create a whole new one from scratch.
Try opening the in game/in ride Workouts tab option.
It brings up the old organizer, but no way to edit. Only option is “Edit Paces” which is just a link to the paces in your Settings. This on latest Win10 client (1.55).