I have been doing a lot of lunging with my horse lately (kind of a re-training process), so was interested when I saw this thread. I am an event/dressage rider and know next to nothing about endurance riding, but wanted to throw in my 2 cents based on what I’ve learned recently.
I used to think lunging pretty much was only good for getting excess energy out, but the dressage trainer I’ve worked with since June is teaching me how to lunge properly :yes:
The amount of muscle (in the right places!) my horse has put on through using proper techniques (getting his hind end under him and used as the engine instead of strung out, and encouraging him to use his back/neck properly) is amazing!! I think you are asking more about lunging for cardio conditioning, and I would think that I would not want to drill a horse around a circle for long enough to really make a strong cardio impact, but I just wanted to make the point that lunging can be really beneficial for other purposes and help with other types of conditioning.
I have learned so much by seeing from the ground how my boy is using himself, and the affects of my attempts (correct and incorrect) to influence how he is using himself. Not sure at all if this applies to endurance riding, but just my latest epiphany! 
ETA: I lunge using side reins, to help as part of the equation to encourage him to balance himself more and not just run on his forehand.