An Australian missionary detained for 13 days in North Korea for trying to spread Christianity has said that he was interrogated for four hours a day and kept under 24-hour guard.

North Korea deported John Short, 75, on Monday, saying he had apologised for anti-state religious acts and asked forgiveness.

Short said that recounting biblical scriptures helped him endure the "long and gruelling investigation". "There were two-hour sessions each morning, which were repeated in the afternoons," he added.

Short, an enthusiastic walker, said his confinement in a room in Pyongyang under constant guard was stressful. "This I found to be most painful physically as an active senior person," he said. "I missed my freedom to walk very much."

Short was detained on 18 February as he prepared to leave his Pyongyang hotel for the airport. He said he admitted his crime: distributing Bible tracts in order to convert North Koreans. "I strongly protested that I was not a spy, nor working with any South Korean organisations nor was I hostile to the DPRK," he wrote, referring to North Korea's official name, the Democratic People's Republic of Korea.

He said he was told that he faced 15 years in prison for distributing religious pamphlets at a Buddhist temple and on a train. "I confessed that I had knowingly broken the law in what I believed is my God-directed duty and as I do in every place and country I visit," Short said. The Korean Central News Agency (KCNA) said North Korea decided to expel him in part because of his age.

North Korea's constitution guarantees freedom of religion, but in practice only officially sanctioned services are tolerated by the government. Defectors from the country have said that distributing Bibles or holding secret prayer services can mean banishment to a labour camp or execution.

North Korea typically frees foreign detainees after they admit their crimes, but many say after their release that their confessions were given under duress. Last week, North Korea presented to the media a detained South Korean missionary who apologised for allegedly trying to reach Pyongyang with Bibles and other Christian materials in October.

Short – who is originally from Barmera, South Australia, but has lived in Hong Kong since 1964 – has been arrested several times for evangelising in mainland China, according to a biography on a Christian website, Gospel Attract.

According to his written apology issued by KCNA, Short said he also visited North Korea in August 2012 to distribute Bible tracts.