...I have w3r3 to do next....
DropZoneOne, I am so excited for you. W5R3 is literally The Best Run! The feeling when you've finished is immense. I cried actual, proper tears upon finishing the run. In fact I started crying about 5 minutes to the end on the run when it occurred to me I was actually going to do this.
No feeling will match it, it's amazing. Even the feeling on graduating and finishing W9R3 wasn't as good. When I first ran 5k non stop, when I first ran 10k, when I first ran 10k non-stop - none of these feelings of euphoria come close to the feeling upon finishing the W5R3 20 minute run.
It really is The Run - it's when you realise I can do this, I'm a proper runner.
Tips:
TRUST THE PLAN!! You can do it. The plan works.
You are going into this knowing it's a long run, so set your pace accordingly. Go slow, slower than you think you need, from the outset. Start slow and stay slow. The most important thing is that you finish the run. It does not matter one bit how fast you go or how far you go.
If you keep your pace low, the only thing stopping you from finishing your run is your mind. Your body can without any doubt physically jog (slowly) for 20 minutes plus. You're legs can do it easily, they really can. If you're heart is pounding or your breathing laboured, you are either going too fast or your brain is telling you to do this due to mental factors, not physical ones.
An experienced runner has mantras like "take control of your breathing" during a run. It's an acknowledgment that panting and heavy, chaotic breathing is a mental thing as well as a physical one. You can focus and control it.
So go slow, know that you can do this and try to relax. The heart pounding is you telling yourself that you can't do this. Change your inner voice to I can do this and you will find your body physically relaxing during the run. Keep repeating it to yourself:
My running is making me fitter
My running is making me stronger
I can do this
I am going to do this
I am fit and strong and will forward this.