Thanks everyone for all your tips - I did the 20 minute run today and it was absolutely fine, just like you've all said!
I went slower than usual from the outset and it was (obviously) a challenging run but Laura breaking it down into 5 minute segments on the podcast was really helpful. When I passed the 15 minute mark it really spurred me on and I picked up the pace a bit back to my natural running speed. Felt really exhilarated finishing the run.
It's amazing what running does for your self belief, I would never have thought this would be possible 5 weeks ago.
Bring on week 6!