Yes over exercising can lead to heart attacks and other problems.
Exercise is good but too much can strain your heart and make it hard for your body to stay healthy.
I walk 20 minutes per day which is good enough to keep me in shape and healthy and I avoid going overboard on the exercise.
If you've watched the news you can sometimes hear how athletes sometimes fall over and die as a result of being out on the field too long and overworking their bodies and hearts especially during hot weather.