During a water-only fast the muscles are used in providing glucose for the Brain and other essential organs, but protein-sparing kicks in along with ketosis, so there should be no reason to be overly concerned about muscle wastage
Strength in the fast.............. http://www.soilandhealth.org/02/0201hyglibcat/020127shelton.III/020127.ch16.htm
If you would like to go below the 170 and reduce your % body fat then Dr Al Sears MD P.A.C.E exercise programme will be your best option by far in my view, as the body remains in ketosis even while you sleep.
We should also bear in mind that the body will attempt to hang onto its fat stores on a water-fast in its efforts to prevent starvation, and why the basal metabolic rate is significantly lowered to this end.