For me I think it has been week 1 and week 8.
Week 1 was a nightmare, those 60 sec runs seemed to go on forever! I was exhausted.
I'm not sure why about week 8 but I had to really push myself to complete the runs, plus I think the hot weather really got to me.
If you click on the Polls button along the top you will see some info on this...
Personally I found week 6 hardest.
Week 6 = tough (a polite word for what I was saying at the time) 
I found weeks 6 - 8 SO hard. I almost gave up on week 8, but as I started week 9 it all came together... Thankfully!
Definitely week 8. I had a difficult first run that week and also the realisation hit me that it was all about just running from then onwards which was a difficult mental battle for me and it took me about 4 weeks to get my head around that.
It's all good now though 
The past 2 weeks when I've been poorly and not able to carry on with the runs. And knowing I was going to have to start from the beginning again. : (
I found week eight difficult also, it was the week that injuries started to show up. Sore knees, sore heels, sore ankles....
That said, week 6 was a bit of a shock. Completion of week 5 builds you up into thinking you are invincible, results in you just trying a bit to hard in week 6.
WOW! Week 3 complete and what a difference
Why so hard and what next? 
Week 8 Run 2!! Most enjoyable run so far!
Week 9 run 2 (and oh what a week it has been)
confession time!
