How DARE you blame the mighty Mizuno Harrier??? Seriously, if it's like what I've got, and it IS tenothingummywhotsit, apparently upping the mileage or overuse can bring it on incrementally, so it kind of creeps up on you. It doesn't have to be a single incident that causes it.
That would fit with me trying to train up for the Stride, Hobble and eventually the Fellsman.