If you want to change a habit, you must also examine an ecology of other habits feeding into or supporting/hindering the one you want to change. For example-not exercising may be linked to watching TV, getting up late, sleeping late, overeating, no true recreation --- the list could go on.
Eliciting these interlinked habits and creating a new ecology will be needed to support the new habit in the long term. One cannot start too big nor can one start too small. What is important is addressing the key ones to bring about change.