You may want to decrease the pace of your walks and the "hilly" challenge to see if you can reduce your heart rate. Overall max heart rate for your age can be found online, and you'll want to maintain 70-80% of that number during exercise. As you mention you have a low resting heart rate, your exercise regimen is straining your heart noticeably, and you want to avoid that over a long period. Depending on your overall cardiac health, you may want to keep your heart rate below 90-100bpm, as suggested. You should also have a detailed conversation with your Cardiologist about your concerns.