Zac Efron struggled to match super-fit Matthew McConaughey

Zac Efron struggled to keep up with movie brother Matthew Mcconaughey during pre-film character-building runs and quickly discovered the Sahara star is perhaps the fittest man in Hollywood.

Zac Efron took on on lengthy running sessions with his The Paperboy co-star so they could work on their characters during intense exercise sessions - and the younger star admits the newlywed left him in his dust, throwing up.

The teen idol says, "Me and Matthew play brothers in the movie... and we wanted to get to know each other really well (and) Matthew's favorite way to do that was to go on long runs and during these runs discuss character and discuss our background and sort of figure out our lives before the movie starts.

"This guy runs so freakin' fast. Matthew is a machine. We'd be on these runs and I'm pretty in shape... but this guy runs with (cyclist) Lance Armstrong.

"He's in such good shape that we're running and I'm trying to keep up and he's having full sentences and very intelligent thoughts about his character and I'm about to throw up... It was insane."