This is a good question - and it is not exactly the same question as "how long should I fast". You're trying to put your body in a state where it no longer has

This is a good question - and it is not exactly the same question as "how long should I fast". You're trying to put your body in a state where it no longer has the resources to support extraneous energy consumers so a too-short fast will not work if you don't cross that threshold of bodily parsimony. I personally would hope that 16 hour IF-style fasts would be enough (because that's what I typically do), but I bet they're not ... But my parents question is a bit different: can one speed up the clock on the fast by burning through a lot of bodily resources through physical work. I suspect the answer is yes, to a degree - but I'll bet cancer cells are not in the same category of energy consumers as your muscles are - I'll bet there is a reserve of energy for them that doesn't get touched for a while, even if you do the big hike. It would be nice if there were some kind of threshold indicator that you could test for to see when your body as in a state of parsimony with regard to cancer cells ...