The side effects are that you increase the efficiency of your lungs so you can do a lot more with less effort. Pushing yourself to improve slowly, but surely is something many people with COPD find difficult. Just like everyone else who exercises, you search for that sweet spot where you're huffing and puffing, but can keep it up without killing yourself for as long as you want your workout to last. So absolutely, increase your time. Do it slowly, but surely. The results will be well worth the effort.