I think that blondie manages to get a renewed strength as he sees the carragie as a way out. He knows that he is more than likely going to die and as soon as he sees this "way out" it gives him motivation to "get his a$$ moving before Tuco gets back with the water. There may be a gun in there that he can get his hands on and turn the tables on Tuco. Surly those two things would motivate a dieing man...

Secondly I think that Tuco and Blondie are "blinded" by greed and the thought of the Gold that makes them un-aware of Angel Eyes comming up behind them.

Regaurdless the movie is still awesome wouldn't you say Kermit?