[wayner]

Somethings I’ve certainly felt in times of being in better shape than others are that:

1) I can break into a sweat quit easily when in shape (preventing over heating)
2) my body becomes more efficient at releasing energy from fat more quickly allowing me to sustain higher levels and longer durations without hitting the wall compared to when in less shape


Of course when you are really out of shape none of this matters. Since your heart and lungs are so in efficient at that point that you tire quickly before ever over exerting your muscles or depleting glycogen stores