You may not feel you are pushing too hard, but it seems your knee thinks otherwise. Walking several mines is a lot for 8 weeks post op and it seems this pain is telling you that. Just because you can doesn’t mean you should.In general, my function is good, I can walk several miles, climb 3 flights of stairs, I feel like my function is where it should be, and am not pushing too hard.
Try backing off on your activity for a while and see if your pain settles down.