Wk 4 run 2... Male aged 46, over weight (duh!) anyway.... half way through and running on flat medium to soft terrain and generally feeling good, mild niggle subsiding in right calf. Then TWANG! Back of left knee goes from super dooper to super painful and I have to stop and hobble home. This was 4-5 wks ago, have rested and only done walks then last week tried a couple of runs and bang on run 2 it did it again. I stretch before and after so it's not that. Does it need more rest or physio or does it mean that running is not my thing?