Question 093: Treating injuries

I am a 49 year old male who has decided to study TKD. The main reason I chose TKD was because they also have a cardio class. I have had one knee operated on twice for torn ACL and meniscus damage but my doctor says I am okay to exercise, but he would prefer something with a lower impact. My question is in regard to progress to higher ranks. I am doing cardio to improve my stamina but I have no flexibility. What should I do to improve it? Also, any suggestions on treating bruises? My forearms are tender for days after self-defense training.