~4 of those miles are easy. If you can run 8 miles, you can train yourself up to 20-22 no problem at all, which you've seen with your 16 miler.
I won't lie to you though, the last 10k are horrid. Maybe you'll make it to 22 and it will only be the last ~4 miles, but you will feel awful. It's nothing to be scared of; it's part of the experience. But you will hurt in a way that is unlike any other. Ask mid-distance people who have run marathons or ask ultra/tri guys, they'll tell you that the 26.2 is the hardest race.
I've done the distance before and I used to coach running, if you have any questions, always happy to chat running.