I didn't set out to change my diet or even diet+excercise. I set out to excercise alone. I changed my diet long before that, but it didn't lead me to excecise and it I wasn't able to do it long enough for it to be ingrained. So my advice would be to excercise even if without a diet change because you are more likely find your diet change as a byproduct and actually be able to sustain it.