I am good at pushing and pushing even when I am weak. So I'd just be very slow if I ride too much, but if I decide I'm not gonna quit, then I'm not gonna quit. Unless I am hungry or sleepy - that is just dangerous and bad for you healthwise, and also very painful.
But anyway, my commute is only 20 miles (round trip), so that comes only to 100 miles a week. So just commuting alone isn't going to tire me even if I do it for years on end.