We have a 40' round pen she uses for developing control with a lunge line. But we have a new young horse that has been quite a handful and it's helped being able to run him in a larger circle that's easier on the legs. It's also a way to give our older horse some exercise without having to ride her. I'm not the horse person, so I don't claim to understand the details. But I know all the neighbors are already over using it, so it must be useful for something.My question begins with what is the purpose of this 80 foot, as you call it, "Round Pen"?
If it is really to be used as a "training ring" for horses where you plan to do ground training on a lounge line, free lounge training without a line, and saddle training for leads, turns, backing, cut backs, etc, you may find it to be too big.
If you are working with untrained young horses you will find that a ring beyond 50' in diameter, particularly for the ground training parts gives them control over you instead of you over them. Not a good way to start a horse because it is much harder to untrain bad habits to retrain good ones than it is to start out correctly at first.
If you are thinking about training you might want to consider killing the grass, at least around the circle next to the insides of the gates and trying to level it by filling any holes, etc. Sprained ankles and pulled tendons on a horse from stepping in a hole at a trot or canter can lay them up for some time.
I've spread wood chips around the outside, they're free and it's easy footing for the horses. We lock them in the round pen for a few hours and they eat the grass down to nothing. She's got an area where there's no grass to teach tricks, otherwise it's hard to keep their head out of the grass.