Can someone give me advice on the diet im starting tomrow?

so basically i plan to not eat for 3 days only drink water to waash out my ssytem, THEN start eating healthily only vegetables and fruits allowed and water obviously. Small portions. I weigh 70-75kg (it keeps going up down but always in under 75) . So what i dont understand is how i find out whehter i've lost water weight or actual fat, cos losing water weight is useless, its not like its long term. Then im going to also do an hour on the treadmill each day at 5.5 mph starting tomorw. How long do you think before i get down to 60kg? Im hoping by the end of august if i work really hard and exercise really well and lead a healthier liefestyle cos i eat a lot of unhealthy stuff and always feel terrible after :(