Krugman points out one benefit of the wearable fitness trackers that are getting popular but misses another. The immediacy thing has been doable for some time now in the form of pedometers. What the newer electronic devices that couple to apps on on your phone gives you is not just the inability to fool yourself but also introduces a game element, including competition with yourself and others.
People [generally] have more fun running 5 km in a race than they do on their own. If I walk 15k steps a day: so what? If I'm involved in a work-week challenge with friends and family: I can win! Also, this adds the guilt element: other people will know if you're a slug. The more automated the tracking and notifications are the more likely people are to do them.
I am reminded of the fitness program in "Ready Player One". He was a slug who did little exercise and ate pizza, but the program, which could not be uninstalled for a month (iirc) wouldn't let him online until he had exercised.
We generally recognize that fitness is more fun when it is a game, but not all people are going to be good/capable of all games. Personal trackers that monitor steps, fitness level, heartrate, allow anyone to play the fitness game.