See you guys are going about it the wrong way. You want to train and run and stuff like that. The secret to being able to last in a long tournament is to be a couch potato. And before you laugh think about it. Who else can sit for hours doing nothing except watch TV or play video games. They only get up to get something to eat and drink and pee. Couch pototos should be your fatigue coaches. They are pros at conserving energy. And in the long run they can outlast you at a table.
The problem with this theory is that you don't have to use your brain while watching TV or movies. You just sit there and either get board with what you're watching (change channels) or get into it somewhat.
Playing a lengthy poker tournament, live or online, is different. You have to pay attention to how the other players at your table play. You have to mentally calculate the
odds for each hand based on both the cards you're dealt and the other players play. Additionally, the cards themselves do form patterns despite online randomness generated by the servers. (chaos theory)
Think about it. How many times have you seen suited A/K, a strong starting hand, turn out to be worthless based on the flop and turn? If you are aware of the pattern of the cards (esspecally in live games), you can maximize your winnings and mimimize your losses.
I once played in a live tournament (amateur league) where the best cards to hold and play were Kings or Fives on one of the tables. So if you were dealt K/5, suited or not, it behooved you to play the hand (normally I would fold them). I was able to use this 'observation' to my advantage and became the chip leader (on that table) because of it. (went out fourth after moving to a different table).
As to mental fatigue, I battle that by trying to have plenty of rest, eating well (but not to much which tends to make one drowsy), and minimizing distractions. This is why I prefer to have my computer in a separate room from the rest of the household, not that this is currently the case as my daughter moved back in with us temporarily, and thus this computer moved downstairs into the dinning room.
Advantage: close proximity to kitchen for food and beverage; next to back door so I can step out to smoke without having to sit out.
Disadvantage: Major distractions from other family members, and the TV in the living room, which I can hear.