Welcome Christine. I don't know a lot about heart rates and pulses, but I think that walking is the best exercise there is. What could be more natural and expected of the human species? If you can walk at all, I suggest taking it slowly. Enjoy it. Don't try to do too much, but do a little everyday.
Also, start tracking your food. You will soon know what to avoid. Make sure you get the calories, protein and carbs you need. Try not to go over.