Everything @Garden Peas says, X2!
Free ranging is wonderful if the circumstances are really right for it, and I hope your environment does turn out to be perfect!
You don't mention how much land you have, but some of the peapeople here have 50 to 100 acres and more and they still need to be constantly monitoring their peas. Peas are so so nosy, and will wander really far just to check things out. Also, as GP says, the "herding back into the pen" thing doesn't usually seem to be effective, although there are exceptions. Peas love to sleep in a spot of their own choosing, preferably high in a tree!