Luckily for me, I am :)
Should have been friends with Tim Chichester.
If they start out slow and maintain a steady pace they are slowing down due to heat. If the run the first half in 1:30 and the second in 1:45, THEY BONKED.
How do you know they actually bonked? They could just be going slow because of the heat.
I think it's a stretch to say that you know they've depleted their glycogen stores.