I'm pretty new to running myself. I took it up late in the winter to start "cutting" from a bodybuilding type of bulk. But somehow, slowly but surely, running seems to be overpowering my interest in lifting. I've got terrible genetics for it, but as a recovering opiate addict, I'm definitely under the impression that running packs a more powerful endorphin punch than lifting.
At the moment, my idea of a "long" run is about 7 miles on a treadmill (1 hr @ 7 mph). The first 45 minutes of the run is "comfortable", but I seem to notice that towards the end, my form breaks down and I'm sucking some pretty heavy wind. And the next morning I feel like I've been hit by a truck (most likely from running ~15 minutes with terrible form).