If I were you and had the money, I would earn a second degree in exercise physiology and earn the ACSM HFS credential.
California University of Pennsylvania has an online program where you can earn your Masters Degree in Exercise Science and Health Promotion. I believe very strongly you should...
1. Find out what the cause of the knee pain is.
2. Get it resolved.
3. Go back to doing the cardio you enjoy.
4. Make sure you avoid what caused the knee pain.
On another note...
I personally enjoy Kangoo Jumps, the take the stress off my knees.
A rotator cuff injury doesn't preclude one from engaging in physical activity.
Your physical therapist will likely prescribe exercises to help strengthen your shoulder girdle.
Too, one has to take into the degree of the injury. I would wonder whether you have a tear or a rupture. Whatever th...
For some it is intrinsic. They are natural movers. They enjoy movement. For others it is extrinsic. They do it because they were told to do so my someone or the individual might be preparing for a wedding or a big race or they feel they can get some type of benefit from it.
Whatever the reaso...
I would honestly go for the bachelors.
It seems to me that you have already satisfied your general electives so at least my from perspective you can knock out another bachelor's degree in two years or less.
Best to you.
Safest approach is speaking with your physician, a sports doctor or a physiatrist.
If this is the route you take, I would also ask your doctor to give his opinion of the "Elliptigo." This might be a good option to running.
Best to you.