According to scientists, the “imprinting” of a habit can take as few as 18 days and as many as 254 days, with 66 days being the average to make the behavior automatic.
A habit grows by repetition and is imprinted in your neural pathways. Your brain changes.
Just as repeating the act builds the habit, missing an exercise session weakens it. I know that some people feel that it they don’t give 100% effort, the exercise session is “wasted”. I disagree. The lighter exercise session may not build more strength or endurance, but it helps to keep the habit intact.
Habits free us from making decisions by creating and nurturing positive, almost automatic actions.