Why is Shinnecock harder on your back than a typical cart course?

Two reasons: walking volume and terrain. A walking round of 18 at Shinnecock is closer to a five-to-seven-mile hike on uneven, sloped ground than a stroll. Add the rotational load of 70+ swings and the spine, hips, and posterior chain absorb roughly an order of magnitude more work than the same round in a cart on a flat course.