Surprisingly, when it comes to stamina and general energy, I couldn't tell the difference.
I don't practice any sport consistently, from time to time I'll join someone for a long jog or go hiking or whatever, but only a few times a year. I tend to walk a lot though, as I don't drive.
Maybe there were changes but the lack of being regular and measuring my performance prevented me from noticing them.
The big difference was more about the upper respiratory system, with huge improvements in terms of taste, snoring, and the throat. I wasn't coughing much when I smoked, but I had a significant amount of mucus, especially in the morning, and had to clear my throat often. I'm also less prone to headaches if I drink a bit less fluids than I should, I feel dry but without the pain.
Well, your respiratory system is probably much healthier now. I would expect better oxygenation to make you less tired, to give you more stamina.
I've never been a smoker, so I'm curious, how big an impact would you say healthier lungs had in your life?