I have a story about the "mind over body" thing.
When I started doing cardio, I did 20 minutes, thought I was very tired so I didn't bother more because I didn't want to exhaust myself completely.
Next time, a few days later, something was bothering me so I started doing cardio on the same bike to think about something else... I did 80 minutes this time... how the heck could I quadruple the time with about the same intensity just a few days later? Take into account I was completely untrained when it came to cardio work.
So I came to the conclusion that my mind gives in A LOT quicker than my legs.
I hate to jog, after 10 minutes I feel like I'm done. But I know that's only my mind trying to convince me to stop, because it doesn't like the activity. Of course, jogging is a lot tougher than a cardio bike, but still.
I realize a smoker might have issues with his respirotary system that I'm unqualified to talk about but... I still have this feeling that your respirotary system is capable of more than your mind leads you to believe! The mind is your enemy, really, in most things. It's devious as well. I wouldn't be surprised if you're going to quick sometimes, just because you unconscienciously want to quit, and therefore fail to pace yourself because that gives you an excuse to just go home
Our bodies can endure a lot of ****. Let me put it like this. Hunters/gatherers in the stone age are estimated to have taken 25 000 steps per day on average. Modern, physically active humans average around 10 000. Inactive modern humans average 1000-2500 steps. Yeah, these numbers are vague, but potentially, you have most of our sociecty taking 10 to 25 times less steps today, then they would've done some thousand years ago.
Oh, and yesterday I saw a women swim from Florida to Cuba or something on TV. Set a record. It took her 24 hours IIRC... 24 hours in the water swimming... that sounds incredible doesn't it? Just goes to show how much our bodies are capable of!