Fergster, my maths is awful, but I have a Casio calculator that has a time calculating function. I find it dead handy and it was cheap as chips from Amazon. It's an SL-1100TV.
Westy, your injury sounds very similar to the one that stopped me in Autumn 2013 campaign. I thought at the time that it may be a stress fracture, but apparently not.
Some sort of peroneal problem or compartment syndrome - I never really found out. For me it was agony to go from walking to running, then abated a bit. Time was the healer and then I ran Mesocycle 5 to get back into running. Frustrating!