All runners get the same questions. Nowadays I just give the same one word answer as I'm bored of the explanation. Along with "Don't you ever get bored?" and "Have you run "THE MARATHON"?" this is one in a series of questions that just make me wish I didn't know people who don't run.
The offending idiot usually believes he has won the argument by saying something like "Well, my mate Dave used to do running and now his knees are knackered, so therefore running is bad for you". You'll not normally here about all the football Dave played, his weight and his tendancy to run with injuries on roads.
Studies cited here suggest that runners do not suffer athritis more than average and that running can help protect the knee rather than destroy the cartilage.
The studies are here. I don't really care, I just wished people would stop asking..