[diffeomorphism]

> Would this eventually lead to 8 productive hours?

In same way that sleeping 10 minutes less each day leads to you not needing sleep at all or showering at 1 degree more each day makes you impervious to boiling water. So, no, absolutely not and potentially dangerous to your health.

[tr1coder]

By the same logic an endurance runner that trains to run 1 km more every few training sessions would end up ...running indefinitely?

[diffeomorphism]

Your example is not quite as obviously wrong depending on your definition of "running" and "indefinitely". With a bit of hand waving I would personally count hiking all day, a marathon also feels long enough to me to count as indefinitely. Furthermore, cross-country marathons are a thing, see for example the movie "Forest Gump".