As an ex-smoker of 8 months and a MTB'er of 7 months, I have found that, when I'm going up a big climb, my legs do not hurt (relatively speaking), but I have trouble keeping my heart rate down, and keeping my breath. My max heart rate is about 198bpm, and it gets unpleasant when it goes over 187bpm. I can hold between 180-185bpm for most of the climb.
This leads me to believe that my respiratory system, is lagging behind my muscle system (if you know what I mean).
I'm 36 with just under 1000 miles under my belt (on and off road, but on a MTB), and I could do with losing about a stone.
Any suggestions how I can improve my respiratory system, I have tried running but it takes to much out of my knees.
Cheers
Dave