Well... Dr. Leddy used the words "turned the corner" today. The appointment was very encouraging. My treadmill numbers have been climbing steadily. I actually went for 38 minutes and 40 seconds last night! The key is not the amount of time spent on the treadmill, though, but my tolerance to symptoms when my heart rate increases. The plan now is for me to go 20 minutes a day (if I can) at the same low speed and no incline and record the highest my heart rate goes. Up until now, I've been just going until my heart rate reaches above 115 and then stopping before I aggravate my symptoms.
So, with this new plan, I should see my maximum heart rate decrease over the next month while I do my 20 minutes on the treadmill. The point is to push the heart rate that I start to show symptoms at. When I see him again in a month, we will reevaluate and see if we can increase the intensity. He said it should be a slow progression for me and I can't push it.