Clover, two big pieces of advice from me - I graduated last year. First, trust the programme. Nearly EVERYONE thinks at some point that there is NO WAY they can do more next week! But you totally can. Just try it, and if you totally fail, try it again, and if still doesn't happen then, and only then, maybe repeat the week before. But otherwise, just keep at it. C25K has successfully got people far less healthy and less motivated than you running 5k/30 mins. If they can do it - you can do it!
And my other bit of advice is do not fret about speed or distance. You can not run too slowly, some people even run slower than they walk. As the times ramp up, just concentrate on running for as long as Laura tells you to. The distance and speed will come later, but if you try to do too much too soon you will struggle. It's the times that matter as they build your stamina and fitness.
Enjoy hun, best thing I've ever done!