You should not be "running" a horse at all on a lunge line. Cantering is very hard on the horse's joints. You should be doing trotting and walking mostly, with just short bouts of canter, like 2-3 laps once or twice each direction. It is very helpful to teach a horse to walk on a lunge line as well.
To minimize stress on the horse's joints, I limit lunge work to 10 minutes each direction, so 20 minutes total. Working the horse in side reins will help build the proper muscles. If you've never used side reins, enlist the help of a trainer or more knowledgable horse friend. Done wrong or too tight, and you can end up with a train wreck...
If your horse is in need of exercise, and is trained for riding then you should be riding. Lunging is hard on a horse's joints as it is, and running an out of shape, overweight horse around and around in circles MAY shed a few pounds but it WILL cause more harm than good. So I strongly suggest some wet blankets and a sore bum for you too, friend!
Yea go ridding! Go out on trails and get some hills in while riding. Lot's of hills!!!! Hold of on cantering for a while unless done in short spurts like directed above. Do lots of trotting and walking and transitions!