Good luck with C25K Do you have a 5k run you can join at the end of the program?
Not to put you down in any way, but running in itself never gave me any significant weight loss (on the contrary: I ate a lot more and gained a bit) but it might be different on 5:2, probably is.
From http://www.examiner.com/article/why-runners-gain-weight
For every gram of glycogen stored, the body also stores 3-4 grams of water with it (Lyle McDonald of Body Recomposition). Runners store more water than less active people, so they may weigh more even while carrying less fat.
But, trousers never lie!
Take care if it's icy!