LONDON (Reuters) - Alan Johnston, the BBC correspondent kidnapped and held for nearly four months in the Gaza Strip, said the mental games he was forced to play to keep despair at bay made his mind stronger than ever.

For the 45-year-old, the first days after his capture in March by the al Qaeda-inspired Army of Islam were the worst.

"You no longer have any power over any corner of your life, almost, and that's a shocking state in which to suddenly find yourself," he said of his confinement, most of it solitary.

"Not only that, but you find yourself for the first time in your life with acres and acres of utterly unfillable time -- the dreadful expanse of ... a whole day of either sitting in a plastic chair or walking up and down a room."

Johnston, who has written a book about his experience called "Kidnapped And Other Dispatches", thought it likely he would be in captivity for a long period, perhaps even years, and engaged in a psychological battle to prevent meltdown.

"On the 11th night I will always remember suddenly emerging into a calmer, more philosophical mood in which I was able just to really begin to focus on what years in that place might mean and how I might get through it.

"Really, that kind of captivity for me at least was like a mental gym. Every day I was working on my state of mind ... I felt towards the end in more control of my mind than in fact I perhaps ever had been.

While stressing that dark times dominated his 114-day ordeal, and that psychological battles were interspersed with long "neutral" periods, he emerged from captivity in July in surprisingly good shape.

"If some people felt that I was composed to a degree when I came out then it was partly that, I was in strangely good control because I'd had to work at it so hard."

MUHAMMAD ALI IMAGE

One way he controlled his mind was to imagine himself as boxing legend Muhammad Ali during his famous 1974 fight against George Foreman.

"The story of that fight really is Ali ... on the ropes for about nine rounds, but big George does begin to tire and Ali comes out dancing, and I used to hope that maybe (in) one week, one month, one year my kidnappers would tire as well."

Another key part of his survival strategy was not to hate his kidnappers.

"To be lying there hopelessly, pointlessly eaten with loathing for the guy in the next room is not going to add to the experience in any useful way," he explained.

During his captivity, Johnston was forced to make a video wearing an explosive belt, heard on the radio that he was about to be freed and may have been executed.

He laughed off the video ordeal as "one more crazy day in the Hotel Jihad", a reference to Hotel California in the Eagles song, where guests can check out but never leave.

The reporter, who is about to take up a London desk job for the BBC and has no immediate plans to return to conflict zones, gained a deeper sympathy for those held against their will and even felt for celebrity heiress Paris Hilton when she was released and then returned to jail.

"I did follow Paris' problems," he said, referring to reports of Hilton's time in jail carried on the BBC's World Service radio programme.

On hearing Hilton was released from prison to house arrest, Johnston believed she had "the easier experience of captivity.

"(But) when they ordered her to go back to prison I honestly did, as I lay there on the floor of my cell in Gaza, think that is a bit unfair. She doesn't deserve that."