I think I may have experienced something similar during times when I was ramping my mileage up, having resumed moderately serious road cycling and training in my late 50s after a decade or so off the bike or just commuting. It's an annoying dull ache, which I experienced primarily at night and which sometimes merged into "restless legs," which I also have from time to time.

I certainly don't know everything, but I happen to be a neurologist with a lot of hard miles on an almost 60 yo body and have experienced nerve, nerve root, and even spinal cord compression and damage and i'm all too familiar with what they feel like. This was certainly none of them. It went away on its own, presumably when I reached some level of endurance/fitness/strength and has not recurred, even after major, exhausting, unaccustomed efforts. That makes me dubious of the cytokine theory, at least in my case. Pet theory: I was on a low dose of a statin during this time and never had any diagnosable muscle problems, but I did go off of it (in accord with guidelines) about the time the pain went away. Don't stop yours, based on my worthless anecdote!