"Expanse" always refers to a physical feature, namely lots of open space. But here it's being used metaphorically to say that there is a lot of potential variation in the optimal eating habits of different individuals.

"This great expanse" refers to the infinitude of the unknowable. It encompasses not only all of time and space but also all of what can be imagined, and all that cannot be imagined as well. It is not a means or an end. It is the context of our existence. Ask yourself the question "where are we?" You could answer that we are on the earth and it orbits around the sun. But where is the sun? Well, you could answer that it is in the Milky Way. But where is the Milky Way? Do you see where this is going? No matter how many times you answer, I can always say "and where is that?". The only way to give a definitive answer is to say either "I don't know" or "infinitude" and the two answers have the same meaning.

After rereading the passage, I also wonder what the writer is trying to say: "We each meet our provenance in the act of practice -- or when we trip out the door trying to evade it. Thanks.