I did over a month in a Malaysian immigration detention.
About 40 guys per room, no blanket or pillow, flat concrete bed, one pair jumpsuit, small tea towel to wash/fan yourself, locked inside 24 hours a day except for 5 mins morning and night to rush to the showers. Basically no one spoke English.
Was rough, a lot of torture, grown men simply don't scream like that, would go on for hours each night by the guards as amusement for them, really hard to sleep even after it stops. Think you'd get used to it but you don't.
Never once allowed to call outside, gf filed a missing persons report and the police told her I left court at the end of the day. She was unable to find me.
Got lucky my lawyer was able to track me down, there's people trapped in there for a year without being able to make a call, got two dudes out with a phone call to their family the day I was released.
Completely insane experience, was a jobs program for useless Malays, they couldn't care less about actually deporting people, they'd have no jobs in that case.
Anyway was doing about 10km walking and about 400m elevation in step-ups daily. Pissed at myself for not knowing any real calisthenics besides dumb basic stuff. Couple of Burmese guys in the next cell over showing me a few which helped.
Definitely the big thing along with preparing yourself for boredom.
Going in again I would memorize pic rel