You might have a lifelong passion for horses, and even devote your whole working life to their welfare. But sometimes there is no substitute for sitting on their backs. A few years ago, Dr. Jeff Berk took a vacation from his relentless schedule as an equine veterinarian in order to-well, spend a little time with horses. His next trip to Britain was not to Tattersalls, as usual, but to an eventing course run by two former Olympians. Though Berk was a perfectly accomplished rider, with years of foxhunting behind him,...