Before I began running I watched a show about early hunting techniques (before spears and bows). They proposed we ran an animal to death. A human can run and recover while animals sprint and then need to stop to recover. So we would run, it would run, we would catch it before it fully recovered and it would run again, but not as fast. After many miles of running and not enough rest, it would be too exhausted and die (or lay helpless while we killed it).
I just finished reading Born to Run by Christopher McDougall. He does a much better job of explaining how to run an animal to death, and more. What a great book.
My next book is 50/50: Secrets I Learned Running 50 Marathons in 50 Days. I won this book from Beth at Running My Own Race.