"I love Lynn," Biel told PEOPLE at Wednesday's New York premiere of Summit on the Summit: Kilimanjaro, a documentary following her recent climb of Africa's highest peak. "Lynn is the most wonderful woman. I am so in love with her."
Less than a month ago, Harless was the one singing Biel's praises. From her front-row seat at the William Rast fashion show during New York Fashion Week, she told PEOPLE: "She's amazing. She's so down-to-earth and her family is great."
Harless's esteem for her son's girlfriend shot to a new level in December, when Biel borrowed her treadmill to train for her Kilimanjaro climb, in which the celebrity participants helped local villagers decontaminate their water. "She was at my house around Christmas and there was this crazy noise coming from my gym," Harless said. "[She] had the treadmill to the highest level and was running with a 40 lb. backpack. I want to be her when I grow up!"
Biel, 27, said her training helped, though she had to focus on her breathing at such a high altitude when she was actually on the mountain. "I really had to take it slow because the second I started to get impatient or feel like let’s just go already, I would get nauseated or my head would start to pound," she said. "You really had to respect the process of the mountain."
Added her trainer, Jason Walsh: "If she feels good, she performs better." Walsh has been working with Biel for 4 years, and started training Timberlake about 18 months ago. "Everybody would be surprised by how athletic Justin is," Walsh said. "He's really strong, has a good mentality – works hard."
The documentary airs March 14 on MTV at 9 pm ET/PT.