After a very long ride I have burned far more than I ate — where does that mass physically go?

On your specific scenario, one correction worth making: the body does not supply an arbitrary deficit purely from fat.

On an effort of that length it will use a mix — fat, stored glycogen, and some protein, with the protein contribution rising as glycogen depletes and the effort continues. That is one reason very long events are hard on the body in ways that the calorie arithmetic does not capture.

The elevated appetite you get for a couple of days afterwards is also real and well described. Resting metabolic rate stays raised while the body repairs and refills stores, so the deficit continues after the ride while the intake catches up. That lag is normal and it is why eating well in the days after a long effort matters more than eating during it.

You breathe it out. That is the entire answer and it is one of the most consistently surprising facts in physiology.

Fat is mostly carbon, hydrogen and oxygen. Metabolising it combines those with oxygen you inhale, and the products are carbon dioxide and water. The carbon dioxide leaves through your lungs and the water leaves as sweat, urine and water vapour in your breath.

When the proportions are worked out for a typical fat molecule, the great majority of the mass — roughly four fifths — leaves as carbon dioxide through the lungs. The remainder leaves as water.

So the lungs are the primary excretory organ for body fat. Losing weight is, quite literally, exhaling it.

Which also explains the thing that seems paradoxical about your ride: you were breathing enormously hard for twenty hours, and that heavy breathing was not merely a symptom of the effort. It was the mass leaving.

Two clarifications that follow from this and that people immediately ask about.

No, breathing harder on purpose does not help. The rate is set by how much you are metabolising, not the other way round. Deliberate over-breathing just changes your blood chemistry and makes you dizzy.

The mass you lose in a day is mostly not fat. After a long ride you might be several kilograms lighter on the scale, and nearly all of that is water and glycogen. Fat oxidation over a very hard day amounts to a much smaller number, and the scale figure recovers within a day or two as you rehydrate.

That is worth knowing because it is the main reason day-to-day weight readings are so noisy relative to actual change.

The mental image that makes it stick, if you want one: a burning log gets lighter, and the mass goes up the chimney.

The chemistry is genuinely the same — carbon compounds plus oxygen giving carbon dioxide and water — just at body temperature, in stages, with the energy captured rather than released as flame. We are slow, well-managed fires with lungs for chimneys.