It's pretty obvious that Matthew McConaughey works out ... a lot. And his "Paperboy" co-star, Zac Efron, reveals the dad-of-two, soon three, is a running "machine."

Efron, 24, trained with McConaughey during pre-production of the movie, which also stars Nicole Kidman.

"Me and Matthew play brothers in the movie and we had a lot of rehearsal to do," Efron explains to Ellen DeGeneres on her show. "We wanted to get to know each other really well ... so Matthew's favorite way to do that was to go on long runs and during these runs [we'd] discuss character, our background, our relationship."

But these "long runs" weren't difficult for McConaughey, who blabbed to Efron while sprinting down roads and trails. Efron, on the other hand, had a hard time keeping up.

"This guy runs so frickin’ fast! Matthew is a machine," Efron laughs to DeGeneres. "We’d be on these runs and I'm pretty in shape, I like to be in shape, so I go on a lot of runs, but this guy runs with Lance Armstrong! He's in such good shape that we're running and I'm trying to keep up and he's [saying] full sentences and very intelligent thoughts about his character and I'm about to throw up."

"It was insane," he adds. "So, I didn’t get much done, but he was just really eloquent and awesome on those runs. And he did have his shirt off!"

As for Efron, he likes to take his shirt off as well, but lately, there's no special lady to share his rock-hard body with.

"I'm not [dating anyone]," he reveals, adding, "Dating's kind of hard."

"You always have to be careful about that kind of thing," Efron says of meeting and trusting new people. "Every day is a new experience, so I take it as it comes. Some people I worry about, some people I really don’t have to."

Watch Zac on the Ellen Show above!

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