Ok so, last Planned Run I pulled up after detecting a twinge in my left calf muscle - 33 minutes into a 40 minute run. At the time it didn't feel bad at all, just oddly sharp and a low level pain. Thinking back I feel it was not that big a deal at the time I was running, so I wonder what compelled me to stop. Anyway I am glad that I did. After running, warm down and stretching, the muscle got steadily more painful that night, and the next day I was walking with a limp - That was Thursday.
The weird thing about it is I can do all my usual calf stretching and there is no hint of pain. The pain only happens as I walk and spring off from the ball of the foot. It seems to be a muscle that is used in the spring action and so is used heavily in faster running. So the 'Fartlek' is to blame ;) . I guess the different action of faster running stressed my calf muscle slightly differently and I ended up with a muscle pull.
As I write this post it is 10:30AM Friday, and my calf is much improved. The previous night I treated the calf with repeated hot and cold which really loosened it up well. I think it is a relatively minor problem and I will be ready to run Sunday morning for the 10km race. But no running until I am completely recovered.
Anyway I have learnt another thing - I will use Fartlek training method quite often as I feel it is going to be a huge benefit, but I have to build it up. Once a week I will use some sort of speed play and increase the frequency over time, and try to avoid doing it two days in a row.