Humans can effectively run forever if it weren't for needing sleep, or eventually, needing to replace fat stores.
> Previous estimates, when accounting for glycogen depletion, suggest that a human could run at about a 10 minute per mile pace, which allows existing fat stores to be converted to glycogen, forever. The only limit to our eventual mileage, therefore, is our need for sleep.
https://nikomccarty.medium.com/how-far-can-humans-run-d5c97f...
> Previous estimates, when accounting for glycogen depletion, suggest that a human could run at about a 10 minute per mile pace, which allows existing fat stores to be converted to glycogen, forever. The only limit to our eventual mileage, therefore, is our need for sleep. https://nikomccarty.medium.com/how-far-can-humans-run-d5c97f...