How did it become obese? Didn;t you walk with the dog? To much foot? Or is the dog sick?
I walk around 2+ hours a day on the beach/in the woods with my dog, to keep him(and me) in shape
Anyway, best thing todo is going to the vet, and ask him what todo. When you walk normally like 10 minutes a day and you start walking for hours it will kill him...