Can't run any more....

Has the knee been repaired? Or are you just waiting for it to heal? Permanent problem or temporary?

I have some permanent issues with my back (surgically removed discs, previous herniated discs) along with arthritis in my knees, and my right knee has also had the cartilage repaired in the past when I folded it sideways. Once the surgery healed and the injuries healed I took back up walking. I'll probably never run again, or may never I just don't know. Constant impact of running would be risking both my knees and my back so I simply walk. The other option is to use a knee brace, and join a gym with a pool and attempt swimming. Knees heal just like anything else, just give it time. I also use a Bowflex M5 which is a modified elliptical/stair-stepper machine. But an elliptical would also put less strain on the knees. I'd work with your doctor he/she can recommend something suitable that wouldn't re-injure the knee.

Get in a swimming pool. Seriously helpful during rebuilding of muscles.

What kind of injury?
Bone? Ligament? Meniscus?

I've had three major knee injuries (two permanent) and can cycle a hundred miles in a day but can only run about 3 miles a week.

Without more info you will only get suggestions for what other people can do with their injuries.

i stopped running after my knee injury, and i just walk instead. and powerlift, but thats not really cardio. i was surprised i could squat pretty well without an acl, i've since had it replaced and can still squat great but i can't run anymore, it hurts.