The series involves the presenter carrying out a number of daredevil challenges, including speeding down a zip-wire.
But she revealed that it was difficult to film last year because she was having IVF treatment to give her three-year-old son Zephyr a sibling (or siblings, as it turned out!)
She told Lorraine why she chose to have more children in her mid-40s and the nickname she and Gerard have given her twins' tummy.
She joked: "There's a double bump – the doublé we call it at home. The Michael Doublé.
"I have a little boy, a three-and-a-half year old little boy called Zephyr, and very soon after he was born that mothering instinct kicked in where I desperately wanted him to have a sibling.
"And I was over 40 and I'd had endometriosis which I've struggled with and lots of women do in this country as well."
She added that she realised her chances of conceiving naturally were slim, especially considering her age.
Julia said: "We decided to embark on several rounds of IVF... and I was doing all of [the TV filming] at the same time as well. And you can't share that information, obviously it's a very personal thing."And you just get on with it, but there were some challenging moments when I was jumping on a zip-wire and I'd have to go to the loo to inject myself."
But it was all for the best, she said: "It was completely worth it and we persevered and kept on going. I'm 27 weeks now and crossing fingers that it will all go well."
Julia said that, after taking a few months off to care for her new babies once they're born, she'll return to work later this year.
She said: "I have been taking it easy. But I'm not going to stop working. I'm back to work in September after the babies are born, and next year a new project and a new Walking series. I'm a working mum, a working woman and it's a part of who I am."