Hello Beth and Popsider,
Many thanks both of you for clarifying this, and thanks in particular for the articles, which were v. helpful. I think that either my running pal was mistaken or I misunderstood :) Either way, I can see now that I'll have to look for another explanation for my sudden second wind. In fact, my money is with you, Popsider: I think it must have been endorphins :)