I've heard talk about this "runners groove". MM used to try to convince me that other people could run and not count every second til they stop.
Well, it happened to me yesterday. We're now training for the Brighton half marathon in February. MM said that if I could run for an hour, I could highly likely complete it. I was sceptical to say the least. However, once the first 10 mins were over, I found my groove. It was odd but I just seemed to settle down into it, and ran and ran and ran. I wasn't out of breath either. So, after an hour, I'd run 8k and it was fine. Aside from needing food and a drink, I probably could have carried on. It's nothing short of a miracle.