I know marathon running and excessively long steady state cardio is considered harmful, but where do you draw the line? Is 30 minutes too much to do on a regular basis? Is there a duration that would be appropriate for a primal lifestyle, or is it the steady unchanging pace that makes jogging any significant distance a poor exercise choice overall?

Personally I enjoy jogging, although I've never really been in shape to go more than 30-45 minutes most of my life anyway. I find it gives me time to think and reflect, I suppose a long daily walk would provide the same time for introspection though.

Thanks for any advice or perspective you guys can provide!