I've been a devout stretcher and practiced yoga for years, but I still don't have much flexibility in my legs (particularly the calves/hamstrings/glutes) and the left leg is much tighter than the right. When I do a challenging ride, I'll develop tightness and dull pain (almost like the pain caused by overstretching) in my left calf and hamstring. Though the right leg will feel tight and fatigued, the left leg is always significantly worse.

I'll take any suggestions at this point! Yes, I've had a bike fit. I stretch right after rides and stretch my hamstrings and calves on days when I don't ride and I use a foam roller nearly every day.