Messi made big strides in his fitness a few seasons back, he changed his diet a lot. Since he likes to play all of every single game, he learned how to be quite scientific in his approach to fitness. Messi stays on the field after practice ends, and he stays in shape even in the off season. Although his work ethic is as good as it gets, like most explosive athletes, his endurance cannot match that of players whose body types are better suited to endurance. On the Barca squad, Roberto, Rakitic, Alba and formerly Alves all have remarkable endurance.
Barca has been supremely successful in the second half of games this year. After the opponents tire out. Because Barca makes them work so hard on defense. Because Barca does not have to run as much, because of ball possession.