Mark's stance on this has changed a lot over the years. He used to think that you cannot be involved in endurance running/cycling/triathloning without having a diet/training regiment like the one he used in his day. If you read The Primal Blueprint and early blog posts he pretty much says this. In some of his later stuff, including a podcast from a couple of months ago, he says that it is possible to go (relatively) low-carb and not train excessively like he did, and still be competitive. He also now states that you can do it, but it's not ideal for you. You will be making some health compromises and will not be as healthy as you could be if you followed his Primal Blueprint Fitness plan of moving slowly, sprinting, and lifting heavy things.