I'm no expert at running, having only completed the c25k a few weeks back, plus I run at our local sweatshop running community club, and the chap there says it can quite likely be something in your mind - if you want to run for 25 minutes, you need to tell your brain you are carrying on anyway!
I didn't want to get out of bed this morning and my brain was grumbling at me to stop for the first 2-3k. I just drop the pace a little and carry on - once you get through that, it isn't so bad...
I still suffer from pace issues, i.e. despite having a watch I still set off too quickly (for me), and pay for it halfway through. I haven't yet mustered that consistent pace despite trying to take it easy to start with.
Obviously if you're feeling pain, then that's a different matter...