"It'll mercilessly eat them up and spit them out."
There were also shorter options over 85km and an 60km run on many of the same trails as the 100km race.
He said with more of the route covered by rocks, roots and hills, runners would need to conserve more energy for later in the race.
Mr Charteris said the decision to re-route the course was straightforward.
"The safety of runners, spectators and volunteers is always the number one priority."
The winner of the 100km event is expected to finish in about eight-and-a-half hours. The final finisher is expected home just before midnight.
Mr Charteris said it had been a hectic few days re-arranging the course.
"There's an army of dedicated volunteers putting in long hours to make sure the race comes off smoothly," he said.
