Well, I think you've hit a sensitive topic here. At least among the people I regularly talk to, this is one of the unsolved challenges every one is facing if their horses have 24 hours access to food.
Basically it should be a matter of providing 24 hours turnout on diverse terrain, big enough and meagre enough to encourage lots of movement during grazing. Since such conditions are almost impossible to come by in my part of the world, try to make up for it with long walks in hilly terrain, riding, lungeing, gymnastic groundwork and stationary strengthening exercises like school halt, rearing, or working on the see-saw.
The point for me is to keep the training as diverse as possible (and fun of course), so that even lungeing is not perceived as a tedious chore, but just another type of exercise. And I actually found with our horses that if it is done in a positive way, even lungeing can be appreciated, as I have the feeling that horses like to improve their physical abilities. It is after all something that determines rank and chance of survival in the herd.
Still, I think that lungeing is not so easy to establish positively, but it certainly can be done. You can either use a combination of positive and negative reinforcement, or even better, rebuild lungeing with free shaping or targeting, like starting with something like the Chase the Tiger
If walking with you on the circle has become a highly reinforced exercise, even with a slight flexion or in shoulder-in, then it's basically just a matter of increasing distance between you and the horse and you have got lungeing.
I believe that horses generally like to move, given that it is not in a stressful situation and they are bodily capable of doing it in a healthy way. So moving on a circle has to be taught and practised first I think, before it can be used as a positive experience for the horse.