I'm doing w5r3 on Sunday and I'm not at all worried! Yes, I am weird.
But I didn't think I could run 3 minutes, 5 minutes and there was no way I could do 8 whole minutes... but I have! It's not fast, it's not pretty, but I'm doing it. I doubt very much I'll be able to do the whole 20 minutes but I'll be doing my absolute best and will just keep putting one foot in front of the other until I can go no more.
Relax, enjoy each run, be brave, but most of all just keep moving! Good luck troops ✊🏻