I don't think I have special expertise that you wouldn't get from a book. The only thing I can say is that the extra half mile you do at the end of your workout, after you think you've given it everything you had, probably provides a disproportionate amount of benefit. Evolutionary just-so story: the body doesn't like to waste resources, but if you push it past its apparent limits, it figures the situation is desperate and maybe you need to be a bit fitter to survive.