Ah yes, they sure are different in open fields, aren't they? They act like they're about to die if you ask them to go forward in the confines of the ring, but put the same horse in an open field and it's off to the races!! I'm doing the same exercise with the circling thing. I change from trot to canter and back to trot every 1/2 of the circle. Just keep circle huge so you don't accidentally throw yourself off balance. They tend to do those circles very fast until they get the hang of the exercise.
You just have to see your distance...you don't have to like it.