Well that was easier than I expected - but not easy by any stretch of the imagination - 1.5 => 3 mins is a big jump but at least there are only 2 runs to do!
My main thing at the moment though is the warm up - it just doesn't do it for me and particularly now it is getting colder if I just walk for 3-5 mins I feel more wooden than when I left the house!
Well done on getting through that! Do a few dynamic warm ups before you leave the house. It really warms you up well.
I second what Annie suggests, I tend to do some dynamic stretches before I go out and then make sure my 5 mins brisk walk is really brisk (it is faster than my run I am sure). Well done!
Welcome to the forum and well done on getting started.
This guide to the plan is essential reading healthunlocked.com/couchto5... and includes links to pre and post run stretching.......they are not the same thing.
In W2 you run for 9minutes, while in W3 you run for...........9minutes.
It is the same duration, just differently arranged.
Enjoy your journey
So finally achieved W3R1
W3R1 done - few twinges but otherwise OK
W3R1. Wahey! Overtaken by toddlers ....but not downhearted!
W3R1 - Am I overdoing it?
W3R1 - time is a funny thing. 
