I asked myself this question the other day when I went for a quick run. I decided to go to the trail running through some areas of town I. Fayetteville. I started near Gregg ave. and headed towards the mall area. It's a nice trail section as a good part of it is shaded by trees. A good part also runs along the creek so that provides some distraction.
The point is that I got tired very soon and didn't reach an hour of running. Rather only barely 40 min. I felt tired and one of the differences from my usual 1 hour runs was that I was carrying the iPhone I order to listen to music while exercising so the easy answer to the question why endurance was not as good as in other occasions was the gadget: when I carry it, I get tired more easily.
There was another detail about this run which is the heat. It was mid nighties when I did this running. Yes, it has to be the heat, ratherthan the gadget.