While his heart was set on completing the Year of Faith, Benedict told Italian journalist Elio Guerriero that after his visit to Mexico and Cuba in March 2012, he felt he was "incapable of fulfilling" the demands of another international trip, especially with World Youth Day 2013 scheduled for Brazil.

"With the program set out by John Paul II for these (World Youth) days, the physical presence of the pope was indispensable," he told Guerriero in an interview, which is included in the journalist's upcoming biography of Pope Benedict. "This, too, was a circumstance which made my resignation a duty," he said.

An excerpt of Guerriero's book, "Servant of God and Humanity: The Biography of Benedict XVI," was published Aug. 24 in the Italian newspaper, La Repubblica.

Pope Benedict said that although he was moved by the "profound faith" of the people of Mexico and Cuba, it was during his visit to the two countries in 2012 that he "experienced very strongly the limits of my physical endurance."

Among the problems with committing to the grueling schedule of an international trip was the change in time zones. Upon consulting with his doctor, he said, it became clear "that I would never be able to take part in the World Youth Day in Rio de Janeiro."

"From that day, I had to decide in a relatively short time the date of my retirement," he said.

Guerriero noted that while many believed Benedict's retirement was a defeat for the church, Pope Benedict continues to seem "calm and confident." He said he "completely agreed" with the journalist's observation.

"I would have been truly worried if I was not convinced – as I had said in the beginning of my pontificate – of being a simple and humble worker in the Lord's vineyard," he said.

The Benedict added that while he was aware of his limitations, he accepted his election in 2005 "in a spirit of obedience" and that despite the difficult moments, there were also "many graces."

"I realized that everything I had to do I could not do on my own and so I was almost obliged to put myself in God's hands, to trust in Jesus who – while I wrote my book on him – I felt bound to by an old and more profound friendship," he said.

The retired pontiff spends his days in prayer and contemplation while residing at the Mater Ecclesiae monastery in Vatican City. For 19 years, different contemplative orders took turns living in the monastery with a mission focused on praying for the Pope and the church.

Benedict said that upon learning that the Visitandine nuns would be leaving the residence, he realized "almost naturally that this would be the place where I could retire in order to continue in my own way the service of prayer of which John Paul II had intended for this house."

Among the visitors Pope Benedict receives is Pope Francis, who "never fails to visit me before embarking on a long trip," he said.

Asked about his personal relationship with his successor, Pope Benedict said they shared a "wonderfully paternal-fraternal relationship" and he has been profoundly touched by his "extraordinarily human availability."

"I often receive small gifts, personally written letters" from Pope Francis, he said. "The human kindness with which he treats me is a particular grace of this last phase of my life for which I can only be grateful. What he says about being open toward other men and women is not just words. He puts it into practice with me."

Pope Francis, who wrote the book's preface, expressed his admiration for Benedict and said his spiritual bond with his predecessor "remains particularly profound."

"In all my meetings with him, I have been able to experience not only reverence and obedience, but also friendly spiritual closeness, the joy of praying together, sincere brotherhood, understanding and friendship, and also his availability for advice," Pope Francis wrote.

The church's mission of proclaiming the merciful love of God for the world, he added, has and continues to be exemplified in the life of Pope Benedict.

"The whole life of thought and the works of Joseph Ratzinger have focused on this purpose and – in the same direction, with the help of God – I strive to continue," Pope Francis wrote.