I did my first Week 3 run yesterday, and was struggling by the end of it.

I've found that the advice they give that you shouldn't be too puffed out is not really realistic - even at the most gentle pace, it shatters me. Having said that, I think it is important to pace yourself; I do mine at a gentle jog, I just can't finish the run if I really open it up.

I have found Week 3 a little bit easier than Week 2, I think because Week 3 is essentially 2 sections, both a 90 second run and then a 3 minute run, so psychologically it seems easier than five or six smaller runs.