The fact that you've seen multiple doctors and they have all concluded that it's anxiety is GREAT news because it means it's anxiety and not something seriously wrong.
Keep walking because in the long run that will help with the anxiety. For the palps make sure you are hydrated (not coffee/soda), eat plenty of lean protein and a banana a day will help regulate your heartbeat.
If you ever think you're actually having a heart attack - don't run vigorously to try and bring a heart attack on - sorry, but that is just foolishness. If you think you're having a heart attack dial 911 and wait for assistance.