You're fine. Size 20 is nowhere near big enough to get to Customer of Size territory. It really starts to become an issue when you're about a size 24/26 or larger on the bottom.
The belts are pretty generous too--I'm a size 26 on the bottom and tend to carry my weight around my belly and they either just fit or are an inch or two short for me. You, being significantly smaller, should have plenty of clearance.
That being said, while it's very, very, very unlikely you'll have any issue, I promise you that in the tiny, infinitesimal chance that you do, you will not die. I've flown with both the extra seat and the belt extender dozens of times and it has yet to prove fatal.
In the end, the stress and embarrassment are 95% in your own head. If the belt doesn't fit, just catch the flight attendant's eye and quietly ask for a belt extender. They're all well trained at discreetly slipping you one. Just act like it's no big deal and, amazingly enough, it will be no big deal.