Depends what you want to do. I generally avoid the lowest ring for long lining but I like them for side rein placement. They allow the horse to get their head down more and stretch the back but then you lose a lot because it also allows them to lean on it. I like the middle rings for long and low and stretchy work and the high rings can be used to get them to learn to pick up the shoulder and round up and over the poll instead of just rounding the whole body. For your guy I would start in the middle or try the lower ones if he's not a lazy leaner and work up. What would suggest before you start "driving" him all over the place though is to learn to match his step while leading. Work on matching him step for step in rhythm at the walk and trot before trying to do it in back. It's easy to get unbalanced if you aren't familiar with the rhythm and tempo of your horse and same as under saddle you can chuck them in the face if you get off balance. The other thing you can do is get a good fitting, non rubbing halter and long line him from a halter. If you're doing it just for his fitness and he's already trained he should understand. That will give you more freedom to make mistakes without chucking his mouth and if you want to "drive" out somewhere and then take a break and let him graze or do some in hand or whatever you have that freedom with the halter. Pretty much get out there and play with it. If you're serious about learning to do the other stuff with a horse without physical limitations there's some great books and guides out there about lunging, long lining and in-hand work.
This one is good and very novice friendly. Http://www.amazon
.com/Longeing-Lining-Exercises-English-Western/dp/0876050461I really want this book. Http://www.amazon.com/Training-Horse-Hand-Classical-Principles/dp/1592287069And I would love to get into the classical dressage in hand stuff like this some day. Http://www.amazon.com/Long-Reining-Saumur-Philippe-Karl/dp/1570762384/ref=pd_sim_b_2
I'm still learning as well but my goal is to learn enough to start sporthorse babies in hand for dressage/eventing and get them started under saddle then sell them on to people looking for up and coming competition mounts. I'm getting much closer to this reality and enjoying every moment of it!