So, I found this forum about ten days ago when I googled "week 5 day 3 impossible - looking for some reassurance, basically, having come home near tears for the first time since starting. I've repeated nearly all the weeks at least once, and always told myself it didn't matter how slow I was working through it as it was all progress - but that first attempt at twenty minutes knocked me for six.
I went back to week 5 day 1 and did it twice, pushing myself to go uphill instead of on flattish ground and build up that way. Today I went to go onto day 2 but at the last minute I decided to keep running for the break, and see how far I could get into it - and I did all five minutes on top of the eight I'd done!
Day 3 now feels SO much more achievable - I'm not going to go for it straight away, I'm going to try 15 minutes and probably 18 before I do 20. It may well only be a psychological barrier but I'd much rather hurdle it than smack my face into it!
Wanted to post this for anyone in the same position I was ten days ago - find something that works for you!