Credit: Mike Coppola/FilmMagic

After reaching the peak of Mount Kilimanjaro, Jessica Biel has set her sights on a new challenge -- a little mountain called Everest.

"I would love to trek to an Everest base camp," the actress, 28, told at the premiere of Summit on the Summit, a 90-minute documentary about a crew who climbed 19,340 feet to raise awareness about the global clean water crisis, in New York City Wednesday.

Although she says the Everest trek is her "next goal," it's actually less intense than her six-day trek with fellow actors and musicians in January because the point she hopes to reach stands at 17,000 feet -- 2,000 less than the summit of Kilimanjaro.

"I feel like I could handle that a lot better for a longer period of time, so that’s my next expedition," she tells Us.

The hardest part about these climbs, she says, wasn't the physical challenges.

"It was really mental," she says of her Kilimanjaro climb. The key, she adds, is  "to not let your brain play a trick on you -- to keep motivated -- to keep inspired even when you're feeling nauseous and your head hurts and you're uncomfortable thinking, 'What am I doing here? This is crazy? This is really, really hard!'"

So, has she been able to show off her physical fitness gains during joint training sessions with beau Justin Timberlake?

"No," she tells Us, laughing. "We really enjoy our workouts... It's not a competition."

She says their personal trainer, Jason Walsh, "is amazing" and "does such a good job keeping us motivated."