I believe lunging for the first ten minutes should be warm up / finishing off some extra energy. Then the last half should be a sort of join up (trying to get horse horse's mind on you and not what else is going on around him or her). By this I mean polishing ground work on the lunge line. For instance like asking from a walk to a trot shouldn't take more then five steps to get into it, and a canter to a stop (if your horse can do it).
This method works great for my horse, and I know it works wonders before going on a trail ride and stuff.