The 23-year-old star reluctantly agreed to work out with Jake and tells "The Late Show with Stephen Colbert" he'll never do that again.
"I didn't want to go because Jake Gyllenhaal is ripped, and I'm really competitive," he explained.
But thinking he could at least out-run Jake, he suggested they run a couple of miles on a treadmill after his castmate suggested a one-mile warm-up.
"I'm like... 'C'mon, let's do two', so immediately I'm regretting suggesting doing two miles... And there's, like, a law in the gym that you can't run slower than the guy next to you. You just can't."