Initial Power Decline

I have been training for just over 20 years - never particularly structured. I’ve been big and less of an athlete than my peers and, looking back, feel my historical effort has been above threshold pace when it was aerobic for others.

As I begin really focusing on my my aerobic pace, my power to be at the correct HR has been declining a little bit.

Is it normal to see an initial decline in pace/power due to historically training so anaerobically?