I'm just doing little bits of sprinting as i go on my long walks... I'm enjoying the rapid improvement of my capabilities, But have no intention of ever running for more than half an hour.
I think that won't take me into the long term damage zone while still evincing my survival heritage of being able to run like hell till out of all danger!
Edit: i live in a very hilly area, and make sure that i use different gaits while on different terrains... I'm always interested in how is uses differing sets of muscles.