Start Workout
Choosing Start Workout from the + menu begins a new, empty workout. This is for situations where you have a good idea of the exercises you want to perform and don't need a predefined workout to guide you.
They're also useful if you just want to keep a record of your time on the treadmill or exercise bike etc. Simply log any exercises as you perform them, and click Stop when your workout is finished.
A record of what you did can be saved to your fitness feed, and the completed workout can be stored in your Workouts area too if you want, to be used as a guide in future.
Active Workouts
Once you start any workout (blank or predefined), it becomes an Active Workout as shown by the screen above. The Active Workout's timer starts and will continue to run, whatever you do on your phone, until you click Stop.
Click Add Exercise to log exercises as you do them
You can press the x button to hide the active workout screen and return to the main app to perform other fitness tasks. You can even exit the app and do other things on your phone if you want - the Active Workout is always running until you stop it*.
If you do exit the app and return to it later, a banner will gently pulse if an Active Workout was in progress. Simply click the banner to return to the Active Workout screen:
Once you complete your workout, you will be given the choice to save a record of the workout (in your Fitness Feed), or to discard it:
Note
*An Active Workout will assume you have finished your workout after 3 hours with no activity.