Running

Yeah, sorry- I wasn't clear, I read that article after Steve mentioned it, I was wondering if it would cause the steady rise seen on that graph, or if it were more likely to be me tiring.

My analysis of the situation is overly simplified. But wereas the effect Hippy describes is true, it will plateau out. Looking at your chart, at around 15-20mins.

I was convinced enough to go into runs, with the mindset that my legs will degrade over 26 miles. But my heart and lungs will be stable. Which helps me to push through the wall.