Just go out and walk, don't think "I have to walk because I need to lose weight", do it as an habit.
You don't think "I need to wash my teeth because I don't want them to rot", or shower every day because "I don't want to smell".
We probably could avoid washing and brushing our teeth for a day, without ill effects. But we do it every day because it's so ingrained in our daily routine that our day does not feel complete if we skip it.
It's much harder to break a rule than to break an exception, and routine is much more powerful than willpower.