Um, another thought: were your tires fully inflated? YOu checked 'em right before, maybe even during the ride?

I had a pretty sucky century yesterday - my friend's first. I even thought we were going to bail because I am basically the stronger rider... if I felt this crappy, he must feel worse, eh? I checked the tires after the first loop because when I feel like I'm working too hard, usually it's the tyres. They were *fine.* He was the one who said "we're not done yet, " not me. (Um, usually I'm the cattle prod )
Now, I was getting over a cold, too... but as we took my bike off the car, the tire was seriously soft (yes, flat by a.m.). No wonder those last 15 miles were so long!