This is a lesson running races teaches us too. If you don't do the work the race is going to suck. Just like if you eat crappy your clothes won't fit for long. You have to make that decision to get out there and train. Sure you can miss a few workouts just like you can have a few indulgent type meals but do it too often and the end goal suffers.
What is your favorite fad diet that you hate? Mine is obviously "shake" type diets. Eat some real food! I think they are fine as a snack, fueling, or recovery but to base your whole diet around them is a load of BS in my book.