|"I worked out this morning really hard, |
so I can probably have a cupcake!"
It's a recipe for failure. It's easy enough to eat healthy for a few days, but throw in the added stress of starting a new workout, following a new schedule etc etc and you will find yourself making compromises and justifying "cheating".
FACT: Self control is effortful, and like any "muscle" if you use it a lot, it becomes tired. At least, there is some evidence that suggests this is true. Check out the Strength theory of self control as discussed by Baumeister et al (1998).
So, add new habits gradually. When you decide to change your life, have a full out plan, but don't dive in or you may burn out.