Thanks for your replies after I failed to manage the full 20 minutes at the first attempt.
I took a few days break before trying it for the second time and this time I did it!
It wasn't easy, each 5 minute block seemed so long and in the last 2 minutes I was struggling to put one foot in front of the other but I didn't stop.
I'm not really feeling the high, I think because it's my second attempt I'm just a glad I completed this hurdle.
Now on to week 6 - that seemed a lifetime away when I was just starting week 1