Yep - people wonder why we can't treat ME/CFS, we don't even have decent biochemical markers for "fatigue" vs "energy", beyond trivial stuff like blood oxygenation and lactic acid. Nor are there much in the way of markers which will determine whether a competing athlete is going to have a good or a bad day.

For example, we have a concept of "energy" for which calories is a rough proxy, but there's no particular reason why fighting an infection should draw on the same reserves that running either endurance or peak muscle does, especially as most people operate in a state of calorie surplus, and their respiratory system is more than capable of supplying a bit of extra O2 unless they're severely ill. And yet clearly the immune/autonomic system forces people into a "rest" state in case of infection.

Or another one, there's no particular biological reason for older people to have less "energy". Like yes there's loss in muscle mass and some small drop-offs in the efficiency of various systems, but it doesn't seem like directly compensating for those makes all that much difference.