I recall reading a study on the top Norwegian xc skiers a few years ago. They found that their performances started to drop off when yearly training volumes went over 900 - 1000 hours( 15 - 20 hrs/week). Over this amount, Injuries started to increase and There was more down time due to fatigue and illness.They came to the conclusion that training any more than this was a waste of time and training time was better spent focusing on race specific intensity training. Now, this was training for events lasting 2 hours or less and not 8 - 10 hr IM races.
Fleck
Steve Fleck @stevefleck | Blog
Fleck
Steve Fleck @stevefleck | Blog