Cardio exercise for person with bad leg?

To put it bluntly, I'm in extremely poor physical condition. Overweight and with high blood pressure and a bum knee - I'm a limping poster child for a heart attack before I hit 50 years old. I need exercise, particularly cardio that my leg can handle. Walking extensively is next to impossible. A round trip to the grocery store is eight blocks and I wanted to cut my leg off before I was finished. Running is right out of the question.

The best option I've found is to use an old exercise bike that was given to me. This bike is placed on a tall table, I sit in an office chair in front of the bike and crank the pedals by hand. I've been trying to do this for at least 30 minutes, twice a day. Obviously this is better than sitting in the chair and watching YouTube videos but is this likely to do me any good? My instinct is that anything that gets my heart rate up will help but do you have any suggestion to improve my results.

I have access to a low impact elliptical but it's a twenty mile drive to use it. The little I've used it seemed to be OK on my knee but I'm not sure it's worth it. While the weather is still nice, I'm going to try to hit the public pool and see if swimming laps works for me. Decades ago, this was almost a meditative stress reliever for me and I certainly consider it worth a shot. Thoughts? Other exercise suggestions?

I have an appointment with my doctor to try to get me on track for a surgical evaluation and possible repair of my knee. I will discuss all this with her at that time. Do you have any specific points that you feel I should bring up with her?

Thanks,

TheHomeDweller