Running into trouble

I like to go running a few times a week – just through some streets in my part of London, nothing Olympian. But sometimes I stay in Cambridge for a while (as I am now). And I find, again and again, over several years, that I can’t run as far in Cambridge as I can in London. I have to set myself a shorter route, and even then my ankles end up being sorer than usual.

I don’t know why this should be. Both areas are pretty flat, I don’t have a significantly different diet while I’m here, and I wear the same trainers. So what’s going on? Is the Cambridge tarmac somehow denser and so harder to run on? Is gravity stronger in East Anglia? Or is there some reason this would be psychosomatic?