This article from the NYT talks about the brain's role in fatigue. I found it interesting because I've experienced something similar. After the SIR ride a few months ago, I found myself climbing better - and I think it is all because I found that when I pushed, my hillclimbing was better than I thought. It was as though I'd learned where my new limits were and my body now had permission to pedal harder. I thought it was all in my head, as it turns out, it is - but not in a bad way.