I was diagnosed with moderate COPD many years age. For the past 15 years I have walked on the treadmill 4miles a day at 3.5 miles per hour. But a recent hospitalization with pneumonia and pleural effusion has made it hard to do. I am using oxygen on the treadmill but my level drops to 82 - 85 while walking at 3.3 miles per hour. Is that ok.