I was lanky until my PCS. Since then, I have developed a symptom called 'postural rigidity'. It is when the muscles of the torso tend to spasm or be held tight. I have added about 10 pounds of muscles to my shoulders and back from this muscle tension. I went from a 36 chest to a 40+ chest.

If you want to maintain good muscle tone, you can do it sitting in a chair. Work through the various muscle groups tensing them, holding them for a ten seconds then release and repeat. You do not need to be getting a cardio work-out to keep your muscle tone. Regaining your cardio fitness will be quite easy when you have recovered enough for cardio workouts.

I was a national caliber distance runner during high school. I broke a foot requiring a cast. Later, I tore my medial meniscus requiring an even longer rehab. I could recover endurance easily because my body already had the long term conditioning.

Some simple efforts should help you maintain your muscle tone. Delaying your recovery will have far more negative impact on your life than needing to recover your cardio fitness.

I hope this helps.

My best to you.