Second Update: By the end of week 1, I was back out riding - both gravel and road - or on the trainer almost daily. Two weeks ago, about five weeks in, my range of motion in my left leg, which had taken a deep thigh bruise, started a steady decline and the pain was back with a vengeance. By the end of the week, I couldn't complete a pedal stroke, even in a controlled environment like the trainer, and was taking stairs one step at a time.
Nice to live next door to a top notch orthopedic surgeon. Knocked on his door. Within about 5 minutes, he suggested that I might be experiencing the growth of some bone tissue inside my left quad. Apparently it's a thing and even has a fancy Latin name - myositis ossificans. X-rays at his office Monday confirmed the diagnosis.
Who knew?
Ultimately there may be a surgical remedy, but for now its just stretch and bear it.