I'll probably get some vehement disagreement about this, but I'd advise NOT riding in pace lines. It's just too dangerous, and raises the already high risk of cycling on roads even higher. Whenever I've espoused this view, I have people say, "Well, only ride in pace lines with experienced riders." But the pros bring each other down all the time! In just my hometown I know of so many local riders who have been badly injured in pace lines, and it continues year after year. Now multiply that by all the other hometowns in the country (and world) and I think it's a tragic and avoidable situation.
My opinion is this: Unless you have professional or serious amateur road-racing ambitions, pace lines are not worth the risk. You can still get very fast and fit, and race in time trials. Leave the broken bones, concussions, and spinal injuries to those who are getting paid for it, or to those who are still young enough to think that that sort of thing couldn't happen to them.