I'm not so sure on if our players are unfit, I had heard during pre season that players stats where much higher on the season previous (even those who had come from other clubs). What I would say however is that there is a couple points to notice:
1) - Our use of the bench, we played pretty much with 15 men on Sunday (no offence to Charles) and towards the end of the game it showed. Sloppy errors in both attack and defence crept into our game particularly from our forwards. This has been a common theme at Hull for a while, I cant recall another team leaving 1-2 players unused on the bench. Ligi Sao, for as much stick as he cops, goes regularly through 80 minutes of graft and never shy's away from carrying the ball in. His efforts are huge but its no coincidence to me that a lot of his errors come in the final thirds of a game.
2) - Following from the above, the sloppy errors lead to having to make more tackles, as mentioned we have the top tackler in the league in Lane, it has never been a positive stat for me. More defending = More fatigue.
I wouldn't say our players are unfit, I would say we need to be smarter to help preserving the players energy levels.