Megs and me are currently going with the fact that he got transported back from t' gamestation to the late 1800s - around the time Torchwood started - then not aging or able to die due to the Bad Wolf. made sure he left London (when he says about going abroad to Estelle) to make sure he didn't run into his younger self, then went on with all the rest of what he was doing.

Mind you,. there's also a theory that Lahore was a con job back in the day, or a Time Agency job.