What does the author mean when he describes the visitors as being "made of lean and leather"?

This reference comes in the 12th paragraph of Momaday’s Introduction. Here he describes the old Kiowa men who used to come along with their wives to meet at his grandmother’s house. Imagine the scene as you read it, and you may understand how the elderly Native Americans were “made of lean and leather.” They were senior citizens who had worked hard enough that they were built entirely of thin muscle, not fat. They spent a lot of time outside in the sun and the weather. Their skin was dark and thickly creased, like an old and worn leather baseball glove. These were the people who still remembered the tribal ways and legends. “They were an old council of warlords, come to remind and be reminded of who they were,” the author says. He was lucky enough to be able to listen to their stories.