Aragorn was the greatest of living men, of a kind seldom seen in Middle-Earth, even in the third age. It is improbable that a regular man could run the distance he ran. Far more improbable that an eighty-something man could! But he was of the race of Numenor. Legolas was Elf-kind, that does not die. Gimli was of the hardy mountain-folk, a dwarf. None of these can modern science tell us anything about.
This is not a NORMAL world they live in. The sun is a vessel of light that sails overhead. A star is actually a Silmaril kindled on the breast of Elwing, who bore it to Earendil the mariner.
In reality, the dead cannot linger because of a curse pronounced by living man. No spell can hold a door closed in today's world, and as yet, no cloak is available to shield the body from the sight of enemies.
We don't go to Tolkien's world to read the ordinary. We go to read the extraordinary. Of course, this is not a blank check to superimpose an alternate reality whenever necessary. If the Three Hunters were more hardy in achieving their journey, it is not a
dues ex machina to say this is fantasy. (Even Eomer was astounded at their deed, showing that there are still standards of performance which the Three Hunters surpassed.) It is simply an alternate world where extraordinary people do extraordinary things.
And if you simply must persist in dietary nitpicking, may I point out the unknown factor of the content of glycogen in
lembas?
Recall, without
lembas, Frodo and Sam would have laid down to die long before achieving their mission.