I was talking to a customer of mine in Louisiana that is training for a 126 miler.  He was explaining the whole mass balance thing and art of balancing intake on such a journey.  Too little and you are screwed, too much and you are screwed.  He was saying he has done 60 milers no problem yet has also hit a wall at 20 miles too by screwing up this balance.  Inspiring to talk to someone who does a 20 miler on Saturday and then a 30 on Sunday all in the name of training.
Me, I am a wuss.  I am stoked by my 8 miler this week.  I find the 1st 4 difficult but by the 2nd 4, my mind is in a free place and the miles seem to run themselves.
It seems like an art and science.