After I was involved in a bad accident last November, I rehomed Molly, my German shepherd dog with a friend who started walking her about 3km (2ml) twice every day. She’s just died suddenly of a heart attack, which has made me feel really bad, in case she was being overexercised. Is this likely?All dogs need regular exercise, and German shepherd dogs are an active breed by nature. Although it is usually recommended to start an exercise programme slowly and build up, there’s no indication from what you say that Molly suffered any discomfort at any stage - not even lameness affecting her hind limbs, which can be a problem in this breed because of weak hips. Six months into this routine is quite a long time, so it’s fair to assume that she had become much fitter after this interval.  It’s possible that she was suffering from the condition known as aortic stenosis, which is a relatively common breed weakness associated with German shepherds, and would have been present since birth. Signs are variable, and tragically, in some cases, dogs do die suddenly and quite unexpectedly, with no obvious symptoms.The cause of the problem lies in the structure of the aortic valve, through which blood flows out of the heart into the aorta, which is the main artery in the body. The narrowing of this valve creates a back pressure, which in turn leads to the left ventricle of the heart increasing in size, as the muscle here has to work harder. The heart also beats faster too, and together, these changes can lead on to sudden heart failure.  Don't feel guilty about what happened. You did what was best for Molly at the time, and frankly, with aortic stenosis, she could have died at any stage with you, in a similar way.  The fact that she was fitter may even have extended her life somewhat.