Hey kayabbott,
I am (was) an avid cyclist and about the same age as you. I have had a number of health issues recently, so it is hard to tell which came first, the chicken or the egg. Cardiovascular Disease, statin therapy (which has taken it's toll on me), a new hip about a year and a half ago and most recently a ruptured achilles tendon. Hard to tell what might be the major culprit of my muscle loss, maybe a combination of things. A number of blood tests haven't revealed anything yet, but muscle loss has been a concern for me. Like you, I used to cycle long distances and hope to get back into it once my achilles heals. I have an exercise bike that will surely be part of my rehab. Best of luck to you! Dennis