It's really too hard to tell these from descriptions. It walks and talks like ectopic beats. With you afib who knows.
I think they only way you will get any actionable knowledge on what these are would be through an event holter monitor.
At 18 METs, you're in great shape, especially at the age of 66 :-) Keep up the good work!
You can be almost certain this is PACs (concidering your other post where your cardiologist confirmed that). If you have runs of AF, it can be a sign that your upper chambers are somewhat irritable. It's fairly normal, especially after decades of heavy exercise.
During the runs of PACs you refer to, you have a slightly increased risk of degrading into AF or other supraventricular (atrial) arrhythmias. This is nothing you need to pay attention to, though, because you will easily notice when this happens (as you stated).
Beta blockers have the effect you refer to. It's a tradeoff you need to accept. I found the solution to be 10 mg instead of 25. It takes care of the PACs and my exercise tolerance is almost unaffected. I hate the runs of PACs, they create a great deal of anxiety (which causes more PACs and so on....) Discuss with your doctor to use a lower dosage, and try to find the optimum where you don't feel PACs but you still can do heavy exercise.
Your problem is a very common one. PACs tend to appear during exercise in great numbers (especially if you are afraid of them), causing great fear. When your sinus node is stimulated, so is the rest of the atrial tissue. Our hearts work that way :(
I agree with the other comments - cut back your dosage and see how you get on. Beta Blockers are intended to keep your heart rate down which is what you have noticed.