Well W5R3 wasn't as bad as I'd feared, and I managed to get just beyond 18 minutes before I just had to slow to a walk.
I did manage to find a rhythm for most of the run so am actually very happy with how it went, just need to re-do and find a little bit extra to push through those final minutes.
Thank you to everyone on here for your wise words of advice and encouragement.
Work commitments mean I might not get to try again before Friday now.