I was a life long smoker also and I have asthma besides. While this limits oxygen uptake and capacity to sustain high speeds, I've seen a vast improvement in stamina from those first days on the bike. I don't train systematically but do push the pace a bit and work on climbing. Surprisingly, even with my mello efforts at improvement, I see slow buy steady progress. I pay close attention to breathing as if I were a high altitude mountaineer. This consists of breathing with the diaphragm and through the nose. Google yoga breathing and breathing exercises for asthmatics, which are all designed to improve breathing mechanics. It works.