English Bull Terriers do have inherited heart defects, they can and should be tested for before breeding. You need to ask the vet what grade murmur the dog has. A grade 1 or 2 will probably mean the dog will never show any ill effects, any higher the possibility becomes bigger. If it were my dog, id let her do whatever she can, after all, a dog will be damn miserable if its not allowed to do anything. When the time comes, and she shows symptoms of it bothering her, then thats the time to stop. Thats me though. best of luck.
Dawn.