> It's a win _if_ the barrier to getting sedentary people to improve their health through fitness is time cost and not all of the other costs associated with habit formation. My guess is for most sedentary people the biggest thing is that they manage to talk themselves out of doing anything, or once they start, talk themselves into allowing cheating/skipping rather than drilling in the habit.

I had exactly opposite experience. Time cost is rather major thing. Especially in the beginning, because it makes organizing your day much harder. Add an hour of anything and suddenly you dont have time to shop or cook or socialize or simply for your favorite hobby. Small time investment make it easier to transition and change lifestyle.

And second, allowing myself to cheat and skipping does miracles for my ability to keep the activity long term. Whenever I set hard requirements on myself, following up became impossible in the long term. And since the attitude is all or nothing, it dropped to nothing.

And third thing that does miracle for gaining long term habit is to prioritize pleasurable exercise, the one I actually like, rather then theoretically most effective one that sux.