Need some exercise advice

Basics from when I was about 12-17 i was classed as an elite jnr althete, doing gymnastics, track, cross country, hockey, netball and football at county level, and swimming at a higher level

so obviously was extremly active, didnt really worry about food as it was eat what I want when I want aslong as had energy to perform

during a football match when was 15/16 another girl fell on my ankle during a tackle, when to the doctors who said its fine its just a sprain

turns out it was alot more than that, after a couldnt put any weight on it, 6 months on crutches and surgery to try and correct the damage - that was 2 years ago now

I started running again during the summer after the ok from the surgeon, the joint couldnt cope its collapsed again bits have broken off its generally yucky

so now I have put on around 3 stone as I didnt change my eating habits from the past to fit with my exercise, as I just wasnt used to not doing it

now im not allowed to run/jump and struggled on the cross trainer as the joint pops, im allowed to train with the swimming club once a week as the ankle struggles to extend or turn out right

im trying to get to the aqua fit at the local pool but unfortunatly the only session they offer is half an hour and clashes with my university work

I was wondering if anyone had any ideas of some suitable cardio ideas or general exercise

thanks