Took a tolerance break for a while and when I would run, I would be out of breath so easily for no reason after I stopped.. and then I recently started and I can run so much easier now! I know THC is a bronchodilator...
You're inhaling deeper while smoking than when not. This will theoretically make your lungs larger after extensive repetition. I believe swimmers practice growing their breath span by breathing in and out as slowly as possible. A quick Google searched confirmed your conviction. Medscape: Medscape Access
However, despite the increased lung capacity, you're still decreasing your ability to absorb oxygen due to carbon monoxide binding to your hemoglobin, reducing your ability to run for long distances.
Perhaps the increased lung capacity balances out the lowered absorption rate or even pushes it towards his favor. That's only conjecture though.