Junk food is addictive and there are additives in junk food to make it addictive (so you will buy more).
A Water Fast will clean out the body including the junk food additives that make junk food addictive.
Without the additives stored in the body the addictiveness of the junk food will be much less.
You could go right into a Water Fast without any preparation, it is just easier during the fast if you do preparation first.
So if you want to go headlong into a good challenge, I say - go for it - you can do it!
Upon completion you are going to know that you have accomplished something and that accomplishment could lead you into the realization that you are actually capable of doing whatever you set you mind to doing in life.