[lightcatcher]

I think the study is assigning health benefits to "being a runner" rather than just "running". It's likely that the selected group of runners who had been running regularly for 6 months had been running regularly for much much longer, perhaps 10+ years. I'm definitely not an expert on the subject, but I don't know how many people are in good enough shape at age 60+ to start running, and I imagine most people running at age 60+ started running no later than their mid 50's.

The real thing you want to control for is the health of the runners and the walkers at the time the runners started running.