Terry,
I know how you feel. I was told a couple of years ago to stop doing any excercise at all due to very high blood pressure. I did not know then if I could ever get back to training again as the doctor also informed me that my heart had suffered damage as well.
I started out by stopping all exercise. However, I just couldn't sit there and do nothing so I decided to do something about it. I would spend half an hour or so every day doing my poomsae. Nothing else, just poomsae. After a couple of times of doing that I went ahead and taught myself the next few poomsae higher than my level. I then added a little bit of kicking, nothing heavy or stressful, just a couple of light kicks, then a poomsae, then a couple more kicks. If I felt a little out of breath or my heart was beating heavier than normal I would stop kicking and do another poomsae.
I reached the stage where I was doing all the poomsae I knew followed by some kicking and then the poomsae again. It formed a nice half hour or so of moderate exercise.
Explain to your students what has happened. Tell them that you will still be instructing them but you personally will not be doing too much stressful stuff due to this condition. You can still teach everything you already teach and you can still train, just not as much as before.
After one year of this moderate exercise we got my blood pressure under control and my doctor was surprised to find that most of the heart damage has mysteriously vanished. Who knows, maybe the same thing will happen to you? Maybe a year or so of moderate exercise and low stress on the heart is all that is required to get it back in shape again?
Just because you cannot run around the track with your students or kick for 15 minutes straight does not mean you have to abandon the martial arts. Just slow down and take it easy for a while. Mr Miyagi didn't run around but he still managed to train the Karate Kid.