Once your body gets adapted you will be fine. You can speed up the adaptation period by doing intermittent DF while exercising 12-18 hours in, daily. Also, a keto diet will help your body access energy in your fat more efficiently
Is this common advice? I haven’t heard it, in fact I’ve heard that exercise should be light when dry fasting. Yes light walking is advised, but 10km is quite a walk, and takes a while if you’re doing at a mid-pace.