I personally just focus on the fast and juice calories. I save exercise for metabolism recovery. Plus with water fasting ,which I do, one is not really suppose to exercise much or work up a sweat to be on the safe side.
Since you are doing juice it is safe to exercise but be aware of your calories and hydration level to avoid "shock"
You should only cut 1000 calories from total intake max to avoid stalls/shocks. Keep the calorie level at an average 1000-1200 at the lowest to avoid shock too.
Just a fat burning tip that really works for fasting:
Keeping you activity "low impact" like walking instead of running (high impact) you burn a high amount of fat percentage wise. The intense exercise will burn more calories and take more weight off BUT if you want to lose in the long run try to make your goal to preserve lean muscle tissue with weightlifting and low impact cardio.