Change up your routine. Try to avoid the bad habits at their usual times moving them about from time to time during the day. But stick to routine with the good habits.

This is how you might quit a smoking habit. You take a different route home than usual, have a cigarette at a time that usually would disgust you, etc. But make sure that you stick to a routine for things like eating and sleeping - and in this case exercise. Slowly introduce it instead of all at once. If you find you don't have it in you to do so at your assigned time, don't do something else instead.