I have just done W5R3. I have never at any time in my adult life run for 20 minutes. I set off thinking I might manage 10 minutes then walk a couple then run again, but when the lovely Laura said I'd done 10 mins I thought 'this is OK, I'll do another 5'. Then I just kept going. I wasn't even begging her to say I'd finished.
The improvement in my stamina is the most remarkable thing and although the programme for W5 looks very scary, I found it less of a struggle than W4 had been.