How to add activity after the fact

To manually add a workout (and its data):

1. In the Health App on the iPhone, tap on the Health Data tab at the bottom
2. Then tap Activity and scroll down to Workouts and tap on it
3. In the the top right hand corner of the screen it says add data (or simply +)
4. Tap on that and you can enter a new workout and its data
5. When you are finished tap on add at the top right of the screen.

Be aware though that this only effects your 3 rings on the day in question.

Also see:

How to manually add a workout in the Health app - Apple Support