Running vs. Walking: Which Is Better for Lasting Health?

I'd be interested to read a similar comparison between running and cycling. I used to run regularly but have switched to mountain biking because it's more fun.

By calories burned they seem pretty similar (I would burn more calories running an hour than riding an hour, but I tend to ride longer than I'd run), and biking is lower impact... at least until you crash. But is it better/worse than running in other ways? I'd be interested to learn.