There's a name for the training philosophy most people follow for endurance sports: long, slow distance (LSD). First heralded in 1969 by running coach Joe Henderson, LSD training is a proven method of building endurance by going easy for hours at a time—20-mile runs, 100-mile bike rides, etc. These efforts teach the body (specifically the mitochondria in muscle cells) to burn fat, boosting the density of the mitochondria and getting more blood to the muscles. Which is a scientific way of saying endurance training makes us more efficient.

I'd never tried LSD training because it always sounded boring and because, as a recreational athlete, I don't train so much as recreate. My rides were generally less than three hours, fast, and in the unstructured presence of friends. That changed last summer when I signed up for a French bike race called L'Etape du Tour. It's an annual citizens' race that follows an actual mountain stage of the Tour de France. In this case, I'd be joining 7,000 other riders for a 105-mile slog from the town of Pau to the mountaintop finish on Hautacam, in the Pyrenees, cresting the 6,939-foot Col du Tourmalet along the way.

I had never done a race longer than three hours, and this would take around twice that. Terrified that I wouldn't be able to last, I devoted myself to eight weeks of LSD workouts. Sure enough, I developed endurance like I'd never known. But the explosiveness I'd earned over a winter of hilly skate skiing was gone. On race day I couldn't close gaps or sprint up the climbs. I had plenty of energy left at the finish, but there was no way I could have gotten there any faster. LSD had made me a turtle.

"There's a time and place for some form of steady aerobic training," says Henderson, noting the fat-burning benefits. "But continuously training at that pace is not going to cause any novel stress. And without that positive stress, you can't get faster. It's time spent but not well spent."

Recent studies support this. Last fall, researchers at Denmark's University of Copenhagen ran an experiment on 17 endurance runners whose workouts consisted of about 28 miles per week of LSD training. For four weeks, nine of the runners were prescribed just six miles of distance training per week, plus an additional "speed endurance" regimen—eight to twelve 30-second sprints separated by a few minutes of rest. The sprints, which they did a few times a week, amounted to another four miles of running, resulting in a total weekly distance of just ten miles. The rest of the runners stuck with their normal 28-mile routines. After four weeks, the control group's fitness stayed about the same, but runners in the sprinting group lowered their oxygen uptake by an average of 6.6 percent, meaning that, at the same speeds, they weren't working as hard.

"It's unlikely that in real life, endurance athletes will replace all their aerobic training with speed-endurance training," professor Jens Bangsbo, a 35-year soccer coach and a principal physiologist on the Copenhagen study, replied when I asked him if I could skip LSD training forever. "But there is scientific consensus that the physiological and performance adaptation from intensity training can be retained for long periods—15 or more weeks… Intense exercise seems to be superior to traditional endurance training, with the exception of muscle-fat oxidation."

Of course, that last point about fat is why you shouldn't eliminate all distance training. To excel at endurance, the body must be able to burn fats efficiently, or else you'll bonk once your carb stores are depleted. And for that there's still no substitute for steady distance work. But if your race goals are more ambitious than simply finishing, your focus should always return to intensity. To get fast, you've got to train fast.