Mark Sisson is a fitness author and blogger, and a former distance runner, triathlete and Ironman competitor, author of the book "Primal Endurance".Mark Sisson is a fitness author and blogger, and a former distance runner, triathlete and Ironman competitor, author of the book "Primal Endurance".Mark Sisson is a fitness author and blogger, and a former distance runner, triathlete and Ironman competitor, author of the book "Primal Endurance".