I had 3 years experience before starting my first dry fast. My first dry fast was not intentional. It was suppose to be a water fast but because I was so busy at work, I forgot to drink water. By the time I realized it, it was 6pm. I felt ok so went to bed at pm. That was my first dry fast experience and felt good. My longest has been 5 days. However, it took a few baby steps for me to actually reach the 5 day dry fast goal. Key word here is baby steps.
I’m in a similar boat to you. I’ve started experimenting with DF in the last couple weeks while listening to my body. I’ve been doing 16-20 hour dry fasts multiple times a week (I’m grouping liquid consumption into my daily intermittent fasting window now), and I haven’t noticed anything negative so far. I did one 48 hour dry fast, and didn’t notice much of concern either. I monitored my pulse throughout the fast to make sure my resting heart rate wasn’t weirdly elevated or anything. My thought process is that if I ease into it, my body will become more efficient with it. I’ve heard that your cells have to be tuned and/or rebuilt with better water conservation in mind, and that seems to make sense to me.
For maintenance, I’ve seen suggested schedule of 36 hours weekly, 3 days monthly, and 5 days annually. When I do 36 hours, I stop eating after dinner, don’t eat the following day, and wait until lunchtime the next day to eat again. I break with an electrolyte drink. (I like LMNT but pink salt in water also works well.) A light lunch will be about an hour later.
Regarding your back aches, I’m guessing that you’ve found what your body wants in refeeding, drink water for a few hours before eating.