If your idea of a workout consists, as mine did, of a pleasant run and a few sit-ups, your first CrossFit session isn’t just a shock—it’s like being tased.

Savvy endurance athletes have long known to include intervals and weights in their workouts. But even they usually don’t do them right. What’s lost is intensity. In a 2009 study by Copenhagen’s Muscle Research Centre, experienced runners who cut total mileage and added speed work significantly improved their 10K times. The key difference, said study author Jens Bangsbo, was that the runners’ 30-second all-out sprints were much harder and more unpleasant than what most people will submit themselves to.

FOR ALL THE BENEFITS, though, how far will high-intensity training and pumping iron carry an athlete? Is CFE really an effective way for most of us to train for multihour gruelfests?

Here the science is frustratingly thin. Exercise physiologists say that such studies simply haven’t been done, but scientists certainly have opinions. Several physiologists and running coaches I spoke with agreed: CFE breaks a lot of dusty old rules—which is great—but it also breaks the law.

“The law of specificity says that if you want to be good at something, you have to do that thing,” says Jay Johnson, a former assistant track coach at the University of Colorado and a contributor to Running Times. We spoke in September 2011, not long after Patrick Makau had run a world-record 2:03:38 in the Berlin Marathon. “Here’s the deal,” Johnson said. “No Kenyan has ever seen a kettlebell.”

Steve Magness, a running coach with Nike’s Oregon Project—a program aimed at producing better American distance runners—told me that the latest theories about elite marathon training incorporate a potpourri of efforts (high-intensity work, hill work, strength work) but don’t sacrifice pure mileage. “If you’re looking to run well, you have to have both,” Magness says. Nike Project athletes log roughly 100 miles a week.

Specificity matters for us age-groupers, too, some physiologists say. When you run long distances, your body changes to help you run even farther, says Ray Moss, director of the Molnar Human Performance Laboratory at the Institute of Running and Scientific Training at Furman University in Greenville, South Carolina. Take that two-hour training run. When you’re done, your body starts creating new capillaries around the muscle fibers in your thighs. That increases blood flow to the muscles and allows for better delivery of oxygen and metabolic substrates like fat.

“This combination permits muscles to use more fat as fuel, thus sparing the body’s limited muscle glycogen,” Moss explained in an email. Your miles-hardened bod becomes a more efficient fat-burning machine, and that makes you less likely to hit the wall deep in the race. “The only way to do that is to run, to train at those longer distances,” Moss wrote.

That seems like common sense, and CrossFit follower and veteran triathlete Robin Clevenger would probably agree. In 2009, Clevenger, then 45, planned to train for two triathlons strictly using CFE. She tested herself at that year’s City of Portland Triathlon, and she ran out of gas.