If you end a workout before completing it and you choose the Don't Save Workout option on the pop-up that appears, you can restart that workout from the beginning in your Workout tab.
However, if you choose the Save Workout option, you cannot restart the workout, and the data will be added to your Fitness Journal.
Don't forget that you can always create and complete a custom workout that is similar to the one you ended. To do this, tap on the Custom Workout button on the Workout tab and create a workout that suits your requirements. It will then be added to your fitness journal and Zing will adapt your upcoming schedule accordingly.