Hi Countess
If you have an encrypted backup of your stolen iPhone, when you come to restore a replacement iPhone from that backup, the restored information will include any past Health and Activity data that was stored on your iPhone up until the point at which that backup was made.
You cannot save any Activity data or Workout data that you accumulate on your watch between the time when it last synced with your original iPhone (and that iPhone was also backed up afterwards via encrypted backup) up until the time when you pair your watch to a replacement iPhone.
You will need to manually reset your watch before it can be paired to that replacement (even if the replacement is restored from a backup of the stolen iPhone).
To reset your watch, go to the Home screen on your watch (a single press on the Digital Crown) > Settings (cog icon) > General > Reset > Erase all Content and Settings.
Unpair your Apple Watch and iPhone - Apple Support
Set up your Apple Watch - Apple Support