I'm in the same situation. When I was in I sacrificed one of my bed sheets to use as a trx suspension trainer. tie it to the staircase on your bunk and you got a ton of different exercises. Roll up your mattress and tie the sheet around as a handle and have your cell mate provide resistance. sounds dumb but it was a lot more effective then I thought. Try to sneak a trash bag into your cell and fill it up as a water bag to substitute for weights. order the largest size slippers and cover them with socks to use as mitts. Run with sweats on during rec. Focus on animal exercises and doing leg work like squats, duck walks, ect. I went in to HCCC at 215 came out 6 weeks later at 165. It's all you can really do so might as well do it. Try to trade your food for fruit. and pound water.
Gearing up to go back in in late March so I know what your going through man. Look at it as an isolated opportunity to work on your mental, physical, and spiritual strength. The trx thing works good though.