I should imagine that dogs suffer like we do. I would suggest not to let him exercise that hard, with your daughter, It maybe to strenuous for him/her. He is 35 in our years. Take note of what your vet says and just let him graze gently. Due to the fact that humans have to be slowed down when they get AF I think it would apply to dogs as well. If you can understand when you are in AF walking to the corner of the road (not far) is like doing a marathon, but not for someone who is not in AF. It is exhausting for some people. This is the only way of explaining things to you. You are tired some days and then others you just cope. Some people are worse than others.
Hope this helps you. I do have a dog myself and he exercises gently but he does take us for a walk, but at present I am unable to take him because he pulls and I to have AF.