Please consider that the tracking is slightly different for a workout than it is for an activity.
Tracking an activity
When tracking an activity, click on the green FAB, and 2 options appear - notes and track activity. Click on the green fab again to confirm that you want to track an activity. You are then taken to the tracking screen.
Tracking a workout
When tracking a workout, click on the green FAB, and 3 options appear - notes, track and start workout. In this case, click the smaller blue FAB with the checkmark to go to the tracking screen.
Also, in a workout you can alternatively track an activity when the workout has ended.