Shaker Aamer spoke to ITV News last week via his lawyer in his final interview before his release from Guantanamo.
He said when he got back to the UK he would need to be tested "like an alien from Mars" because of concerns about his physical and mental health.
Mr Aamer said the first thing he wanted on his return was a cup of coffee and then a full medical check-up before seeing his family.
Speaking through his lawyer, he said: "Imagine you have an alien who came from Mars – you want to know what kind of alien blood he has, what alien bones he’s got.
"I don’t want to infect my kids or my wife with any disease so I need to be checked.
"I am an old car that has not been to the garage for years."
Mr Aamer, referred to as prisoner 239 at Guantanamo, said as soon as he was free "the old 239 is going to come back with a lot of issues".
He said he may struggle to deal with being called by his name again.
Mr Aamer said: "239 is my prison number here and they always call me by it. I am not sure I would know to respond to anything else – like my name, Shaker.
"I have known nothing about the real world for more than 13 years."
Mr Aamer thanked those who have supported him and said he cried when he heard they would never stop campaigning for his release.
Asked about his detention at the CIA black site on the Bagram US base in Afghanistan, Mr Aamer said there was "much to come out" about the site.
He said but wanted an inquiry, but added: "I don't want to put anyone in prison."