Cushings Disease and Arthritis in my Black Lab

I had to put my 15 yr old black lab to sleep last year. I know he lived a long life; but, the vet said his heart was like that of of 6 yr old. He had Cushings Disease and arthritis that really debillitated him. He had problems breathing at the end. Has anyone else dealt with an animal with this illness? I have been wondering if it is something common; or, if there was someway I could have prevented him from getting it.