In the early eighties in college I knew many Olympic atheletes, mostly swimmers and track stars. Some of them even Gold Medal winners, and they smoked weed before working out.
I think that most people get lazy after smoking because that is what they are used to doing when smoking. It is a simple fact of habit not cause.
I used to do a lot of manual labor, landscaping and carpentry and I would often smoke when I was feeling tired. I would get a boost, maybe this is from smoking and running all the time. Of course, too much smoking definitely will make the lungs a little less clear. So eat a brownie and go run over a mountain. You'll love it.
Also a little point and fact. THC activated rubbing alcohol is the ultimate for sore or strained muscles. This is one incredible cannibas medicine that is still off the radar. If you use enough of it you will get an amazing body high, with not head buzz and it won't tax your liver at all because of how you intake it through the skin.