Olivia Newton-John has compared her battle with breast cancer to climbing the Great Wall of China.The Grease star has become a health advocate since fighting off the disease following her 1992 diagnosis.And Newton-John - who led an inspirational three-week walk along the Great Wall of China in just last April - insists her efforts have awakened a fighting spirit in herself.She says, "I liken the walk to the cancer journey actually, because it was the unknown, it was sometimes difficult, it was strenuous, it was challenging."But when you got through it, it was amazing. (There was) an amazing feeling of accomplishment... I've never done anything like that in my life."