I have NO experience with Leo's!
I can only tell you that with Beau I was told to be very careful.
Too much exercise is not good for the giants.
I used to take him into the front yard and play chase!
Not me chasing him (didn't want him to run away instead of coming to me especially when older) but him chasing me.
I would go hide behind a tree or rock and yell out "follow me!"
Then he would be all over the place looking for me. I would run when he found me and he would follow. Then of course he got a treat for being a good boy. I would do about 3 of these a day and maybe just walk him for 10 min at his own pace.
So far so good, he's 2 now and no problems.
I found this web site:
http://www.yourpurebredpuppy.com/reviews/leonbergers.htmlProviding the proper balance of exercise. Young Leonbergers need enough exercise to keep them lean and healthy, but not so much that their soft growing bones, joints, and ligaments become over-stressed and damaged. Adult Leonbergers need more exercise to keep them in shape, but not in hot or humid weather for fear of overheating. The proper amount of exercise can be difficult to regulate in giant breeds.
Since you have to minimize their exercise, young Leonbergers can be very rambunctious. They will romp with uncoordinated gawkiness all over your house. You need to substitute extra quantities of companionship and supervision. Otherwise, left alone, young Leonbergers become bored and destructive -- and their powerful jaws can literally destroy your living room.