Jelly, you're not the only one, i done the same thing by rescuing a broiler chicken and even though she was a Ross/Cobb i took her to be ex batt, which just isn't the same[ i think its worse in birds bred for meat ]. When the penny finally dropped i also panicked because of all the treats i'd given her and expecting her to die on me, she was very heavy.
I got lots of advice and got her free-ranging with the others, i still gave her treats but made her come further to get them.
Sadly , she got a foot infection, i bought her indoors, took her to the vets, anti-biotics etc, but she got too bad and i had her put to sleep. It was my first loss and i had devoted so much time to her, she was really special. It wouldn't stop me doing it again as she had a really good couple of months.
Try to keep everything as flat as poss, the slightist jump can cause what happened to Grace. Just enjoy them whilst you can, they can live good longish lives.Try to be strict, its for their own good[ i found it hard!]
Good luck kitkat