Thanks to everyone who told me I was wrong to be worried about week 5 run 3. I just ran for 20 minutes, that's bonkers! You lot were totally right.
If anyone is struggling here are some things I have figured out along the way:
Pretend you're not running- think about something else, try to spot wildlife, focus on the lyrics of your music, practice a presentation in your head, think about anything except how much time is left or how tired you are.
Don't clench your fists, it's a waste of energy.
If you run the same route every time you very quickly learn where the finish line is, but remember you're running on time, not distance. Speeding up at the end will get you there faster but it will also move the finish line and wear you out prematurely.
Personally, I find thinking about my breathing makes my breathing worse, if I don't think about it I breath easier. This may be different for everyone though!
If you're thinking about stopping, think about how happy you will be if you get to the end without stopping, it's worth a miserable few minutes towards the end (but don't run yourself to death).
Assume you can do it before you start and you probably will.