Having it affect the runner's experience of time has to remain part of the core concept. I actually added the "entropy field" part as a reason for why they don't just lock it in a room somewhere and forget about it. Plus, I kinda like the idea of this little hut out in the middle of no-where with nothing in it but a guy on a treadmill, running forever and desperately wishing he could stop while knowing that he dare not.
Hmm… maybe change it to the person completely stops biologically experiencing time while on the treadmill? So they never get tired or thirsty, but never gain any muscle tone or lose weight either.
Or maybe have it where instead of aging you, running on it actually slows your aging process. Theoretically, you could live forever, so long as you could keep running at [[TOO DAMN HIGH]] miles per hour without passing out.