Yup, we have a limit in our ability to take in and metabolize calories, although it is somewhat flexible in that if you do it enough you'll get better at it. Look at the people who do the long thru-hikes. Stuff like the Appalachian Trail (Georgia to the highest point in Maine), PCT or CDT (both run from the Mexican border to the Canadian border). They will hit saturation on the ability to take in calories, although enough time out there can increase the ability to metabolize fat (very useful in that it has about 2x the calories per pound of other food)

But you are right that it's very definitely about the balance.