I have a 4 year old male that wasn't neutered until about 3. He came up with minor hip dysplasia. I see no difference in his personality or drive in the field. He is the same dog through and through!
As for the pacing, maybe he needs more stimulation? Does he pace even after a good run or training session? he might just have some pent up energy that needs to be burned. It also could be a bit of an anxiety thing, do you try to snap him out of it when he's pacing? take his mind of it and give him something to do... Try redirecting him when he is showing the behavior, make him heel around the house for a minute, sit, lay down, here, heel... just give him a few quick tasks to think about and it could help! Good luck buddy!