As I suspected, hills.  I managed three four sets of 100, 200, and 300 metres up the hill, each rep followed by a jog return.  I try hard on the way up, but I have to walk for the first fifty on the way back down, before commencing a really slow jog.  I like hills, but usually I'm just trying to make it to the top without stopping, not busting a gut trying to race up.

Tonight's run is here.