As I discovered I could manage, I've run 5K a few times (36 minutes) despite not quite being at the end of C25K. I am shattered at the end of these. Today I was doing the same but after 20 minutes I felt my calf tightening and stopped early. The really interesting thing was that afterwards I was barely short of breath and felt as if I'd hardly exerted myself (the pace was still 7 min/km and I still felt I was putting a lot into it as I was running). Although it was a shame not to finish the run this seems like real progress. Not only to be able to run for 20 minutes (which I was amazed at when I first did it) but to be able to run for 20 minutes with ease!