By Philip Pullella ABOARD THE PAPAL PLANE (Reuters) – Pope Francis said on Saturday that his advancing age and his difficulty walking have ushered in a new, slower phase of his papacy and repeated that he would be ready to resign one day if serious health problems prohibited him from running the Church. “I don’t think I can continue doing trips with the same rhythm as before,” he said in answers to reporters’ questions aboard the plane returning to Rome from a week-long trip in Canada. For the past few months Francis, 85, has been using a wheelchair, a cane or a walker because of knee pain cau…