Every check-in is a photo. No streak you can fake, no box you can tick without doing the thing. Show up, take the shot, move on.
The whole loop is designed to be over before your motivation gets a vote.
Open the habit, take the photo. That's the check-in — there's nothing else to fill in.
Your run is visible the moment you close the camera. Miss a day and you'll feel it, which is the point.
Add a buddy and you both see each other's proof. Much harder to quietly skip.