After a pretty poor run on Saturday, and a busy week when I couldn't get out at all, I had another go this evening. I did the C25K+ "Stamina" run, which involved running for 35 minutes, at increasing numbers of paces per minute. I did it!
I've just worked out my distance on Mapmyrun, and I did 4.5K. I was just astonished that I could keep going for that long. I wasn't really tired, either - I was definitely thinking "this is a lot of effort" and "I hope this stops soon", but not "help! I can't manage!".
I did a chi running workshop the other evening, and the tips the teacher gave us really helped - notably keeping the head up and back straight, using the core muscles, and letting your feet "cycle" as they move along. It was interesting that Laura was saying much the same thing in the podcast.
It occurred to me that it would have been really good to have this information on technique when I started the C25K course - perhaps the first run of W1 could have been about teaching new people the technique. I felt I spent a few weeks running in a really inefficient way, and I've only just started improving.