Ok, I did something to my ankle yesterday.  The outside arch of my right foot KILLS!  Stuff online suggests that it might be a pulled peroneal tendon – which wraps around that outside part of the foot.  The ankle is sore a bit by the ankle bone – also where the tendon goes.  In any case, I bet that running on the snow is what did it.  It is not just an uneven surface (dirt and trails is always plenty uneven – and not a problem) but it is an uneven surface that unpredictably sinks or depresses under your footfall.  It is hard to gauge what its going to do for every foot fall, so I wonder if the brain has a hard time tensing up the right muscles to prepare for impact.  In any case – I’m totally limping.